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How to Find 5th or ending date of a particular day in a month based on date of the same month

I have been trying to find the 5th week date of a day in a month like 5th week Monday date, 5th week Tue date, Wed... and so on based on the date from the same month. This date could belong to any week of same month.
I tried like
DateTime MonthEventDate=05/01/2016; //Date format in dd/MM/yyyy format
DayOfWeek EventDay="Sunday"; //For Example i want to find 5th Sunday in January Month, but days too will change everytime based on user selection
string SelectedWeek="5"; //Here i'm getting the week in which i need to find the given date i.e, 5th Monday or Tuesday & so on
if (SelectedWeek == "5")
{
//Here i tried to add number of days to my initial day to find 5th day date, but every time its returning next month value
MonthEventDate = MonthEventDate.AddDays((EventDay < MonthEventDate.DayOfWeek ? 31 : 28) + EventDay - MonthEventDate.DayOfWeek);
}
I know the logic is wrong but i want to get date of 5th day of the week, and if that day is not present, return 0. Looking for some guidance
Note: Here Month will change based on User Input, so how to return Date of fifth day, if it exist in the given month

This should give you the 5th day (if there is one) ...
DateTime dayInMonth = DateTime.Now;
DayOfWeek dayToFind = DayOfWeek.Friday;
var fifth= Enumerable.Range(1, DateTime.DaysInMonth(dayInMonth.Year, dayInMonth.Month))
.Select(day => new DateTime(dayInMonth.Year, dayInMonth.Month, day))
.Where(day => day.DayOfWeek == dayToFind)
.Skip(4)
.FirstOrDefault();

Extending #Soner Gönül's answer. In input you put required day of week, required month and required year. On output you get date of same day of week in fifth weeks, or null
public DateTime? FifthDay(DayOfWeek dayOfWeek, byte monthNum, int year)
{
if (monthNum > 12 || monthNum < 1)
{
throw new Exception("Month value should be between 1 and 12");
}
var searchDate = new DateTime(year, monthNum, 1);
var weekDay = searchDate.DayOfWeek;
if (weekDay == dayOfWeek)
{
searchDate = searchDate.AddDays(28);
}
for (DateTime d = searchDate; d < d.AddDays(7); d = d.AddDays(1))
{
if (d.DayOfWeek == dayOfWeek)
{
searchDate = searchDate.AddDays(28);
break;
}
}
if (searchDate.Month == monthNum)
return searchDate;
return null;
}

You can use a simple math like this (no validations included)
static DateTime? FindDate(int year, int month, DayOfWeek dayOfWeek, int weekOfMonth = 5)
{
var baseDate = new DateTime(year, month, 1);
int firstDateOffset = ((int)dayOfWeek - (int)baseDate.DayOfWeek + 7) % 7;
var date = baseDate.AddDays(firstDateOffset + 7 * (weekOfMonth - 1));
return date.Month == month ? date : (DateTime?)null;
}
I think the code is self explanatory. The only trick that probably needs explanation is the line
int firstDateOffset = ((int)dayOfWeek - (int)baseDate.DayOfWeek + 7) % 7;
which handles the case when let say the month starts in Friday and you asked for Monday, and is a short equivalent of
int firstDateOffset = (int)dayOfWeek - (int)baseDate.DayOfWeek;
if (firstDateOffset < 0) firstDateOffset += 7;
Usage in your example
var monthEventDate = FindDate(2016, 1, DayOfWeek.Sunday, 5);

You can refer this demo code
int requiredDay = 5; //Day number i.e 0-6(Sunday to SaturDay)
DateTime day = new DateTime(2016, 1, 1); //Month,year,Date
if (DateTime.DaysInMonth(day.Year, day.Month) > 28)
{
//Get the first day name of the month
int firstMonthDay = (int)day.DayOfWeek;
int offset=0;
//Number of days from the first day for the required day
if (firstMonthDay <= requiredDay)
offset = requiredDay - firstMonthDay;
else
offset = 7 - firstMonthDay + requiredDay;
//
DateTime firstoccurence = day.AddDays((double)offset);
DateTime fifthOccurence = firstoccurence.AddDays(28);
if (fifthOccurence.Month == firstoccurence.Month)
MessageBox.Show(fifthOccurence.ToString());
else
MessageBox.Show("No 5th Occurence for this day");
}

Calculate the Number of Mondays in a Month

I want to calculate the number of weeks within a month.
The first week of January 2014 starting from the first Monday is the 6th. So, January has 4 weeks.
The first week of March 2014 starting from the first Monday is the 3rd. So, March has 5 weeks.
I want to know how many weeks there are in a month counting from the first Monday, not the first day.
How do I do this?
I have this code but it is used to get week number of the month for specific dates.
public int GetWeekNumberOfMonth(DateTime date)
{
date = date.Date;
DateTime firstMonthDay = new DateTime(date.Year, date.Month, 1);
DateTime firstMonthMonday = firstMonthDay.AddDays((DayOfWeek.Monday + 7 - firstMonthDay.DayOfWeek) % 7);
if (firstMonthMonday > date)
{
firstMonthDay = firstMonthDay.AddMonths(-1);
firstMonthMonday = firstMonthDay.AddDays((DayOfWeek.Monday + 7 - firstMonthDay.DayOfWeek) % 7);
}
return (date - firstMonthMonday).Days / 7 + 1;
}

Try this:
Get the number of days in the current month, and find the first day. For each day in the month, see if the day is a Monday, if so, increment the value.
public static int MondaysInMonth(DateTime thisMonth)
{
int mondays = 0;
int month = thisMonth.Month;
int year = thisMonth.Year;
int daysThisMonth = DateTime.DaysInMonth(year, month);
DateTime beginingOfThisMonth = new DateTime(year, month, 1);
for (int i = 0; i < daysThisMonth; i++)
if (beginingOfThisMonth.AddDays(i).DayOfWeek == DayOfWeek.Monday)
mondays++;
return mondays;
}
You can use it like this with the current date:
Console.WriteLine(MondaysInMonth(DateTime.Now));
Output:
4
or with any month you choose:
Console.WriteLine(MondaysInMonth(new DateTime(year, month, 1)))

Just correction to Cyral code :
i must start from 0 as he is using AddDays method .
Reason :the above example returns 5 for NOV and 4 for DEC which is wrong..
Edited code :
public static int MondaysInMonth(DateTime thisMonth)
{
int mondays = 0;
int month = thisMonth.Month;
int year = thisMonth.Year;
int daysThisMonth = DateTime.DaysInMonth(year, month);
DateTime beginingOfThisMonth = new DateTime(year, month, 1);
for (int i = 0; i < daysThisMonth; i++)
if (beginingOfThisMonth.AddDays(i).DayOfWeek == DayOfWeek.Monday)
mondays++;
return mondays;
}

I tried to use that code, but in some cases it didn't work. So I found this code on MSDN.
public static int MondaysInMonth(this DateTime time)
{
//extract the month
int daysInMonth = DateTime.DaysInMonth(time.Year, time.Month);
var firstOfMonth = new DateTime(time.Year, time.Month, 1);
//days of week starts by default as Sunday = 0
var firstDayOfMonth = (int)firstOfMonth.DayOfWeek;
var weeksInMonth = (int)Math.Ceiling((firstDayOfMonth + daysInMonth) / 7.0);
return weeksInMonth;
}
I created it like a extension method, so I can use like that:
var dateTimeNow = DateTime.Now;
var weeks = dateTimeNow.MondaysInMonth();

The faster method without looping
private static double GetDaysCount(DateTime sampleDate, int DayOfWeekToMatch)
{
int FirstDayOfMonth = (int)(new DateTime(sampleDate.Year, sampleDate.Month, 1).DayOfWeek);
int FirstOccurrenceOn = 0;
if (FirstDayOfMonth < DayOfWeekToMatch)
FirstOccurrenceOn = DayOfWeekToMatch - FirstDayOfMonth + 1;
else
FirstOccurrenceOn = (7 - FirstDayOfMonth) + DayOfWeekToMatch + 1;
int totalDays = DateTime.DaysInMonth(sampleDate.Year, sampleDate.Month);
return Math.Ceiling((totalDays - FirstOccurrenceOn) / 7.0f);
}

How to get set of dates in a specific week ,given the month and the year?

How to get The dates in a specific week given the month and the year ?
For example:
My parameters :
June - 2013 - The second week
I want the result set like this :
9-6-2013
10-6-2013
11-6-2013
12-6-2013
13-6-2013
I want it starting from Sun to Thu .

Well in my Noda Time library I would:
Start from the end of the previous month
Loop as many times as you want finding the next Sunday
Go from there, yielding the days (so you can just use a foreach loop):
So:
IEnumerable<LocalDate> GetSundayToWednesday(int year, int month, int week)
{
LocalDate date = new LocalDate(year, month, 1).PlusDays(-1);
for (int i = 0; i < week; i++)
{
date = date.Next(IsoDayOfWeek.Sunday);
}
// You always want 4 days, Sunday to Wednesday
for (int i = 0; i < 4; i++)
{
yield return date;
date = date.PlusDays(1);
}
}
Using just DateTime, I'd probably start at the first day of the month that it could be (week * 7 + 1) and loop until I hit the right day of week, then go from there:
IEnumerable<DateTime> GetSundayToWednesday(int year, int month, int week)
{
// Consider breaking this part out into a separate method?
DateTime date = new DateTime(year, month, week * 7 + 1);
for (int i = 0; i < 7; i++)
{
if (date.DayOfWeek == DayOfWeek.Sunday)
{
break;
}
date = date.AddDays(1);
}
// You always want 4 days, Sunday to Wednesday
for (int i = 0; i < 4; i++)
{
yield return date;
date = date.AddDays(1);
}
}
Looping like this isn't terribly efficient - you could just work out how many days to advance - but it's more obviously right. You can very easily end up with off-by-one errors (or going back into the previous month) with a more efficient approach. You may choose to put more effort into being efficient if this is important, of course.

I have not really tested this but I think that does more or less what you want:
int year = 2013;
int month = 6;
int lookupWeek = 2;
int daysInMonth = DateTime.DaysInMonth(year, month);
int weekCounter = 1;
List<DateTime> weekDays = new List<DateTime>();
for (int day = 1; day <= daysInMonth; day++)
{
DateTime date = new DateTime(year,month,day);
if(date.DayOfWeek == DayOfWeek.Sunday && day > 1) weekCounter++;
if(weekCounter == lookupWeek) weekDays.Add(date);
}

If you are interested in the client-side (JavaScript) route, then date.js might be worth a look, it allows for the following:
// What date is next thursday?
Date.today().next().thursday();
// Add 3 days to Today
Date.today().add(3).days();
// Is today Friday?
Date.today().is().friday();
// Number fun
(3).days().ago();
// 6 months from now
var n = 6;
n.months().fromNow();
// Set to 8:30 AM on the 15th day of the month
Date.today().set({ day: 15, hour: 8, minute: 30 });
// Convert text into Date
Date.parse('today');
Date.parse('t + 5 d'); // today + 5 days
Date.parse('next thursday');
Date.parse('February 20th 1973');
Date.parse('Thu, 1 July 2004 22:30:00');
I find the natural language syntax (i.e. 'next thursday') to be quite powerful.

This method gives the days in a specific week of a month:
static IEnumerable<string> DaysInWeek(int year, int month, int week)
{
var date = new DateTime(year, month, 1);
var calendar = new GregorianCalendar();
var firstWeek = calendar.GetWeekOfYear(date, CalendarWeekRule.FirstFullWeek, DayOfWeek.Sunday);
var days = calendar.GetDaysInMonth(year, month);
var daysInWeek = (from day in Enumerable.Range(0, calendar.GetDaysInMonth(year, month) - 1)
let dayDate = date.AddDays(day)
let week2 = calendar.GetWeekOfYear(dayDate, CalendarWeekRule.FirstFullWeek, DayOfWeek.Sunday) - firstWeek + 1
where week2 == week
select day + 1).ToList();
foreach (var d in daysInWeek) yield return string.Format("{0:00}-{1:00}-{2:0000}", d, month, year);
}
And the output of this:
foreach (var d in DaysInWeek(week, year, month).Take(5)) Log.Info(d);
Would be:
09-06-2013
10-06-2013
11-06-2013
12-06-2013
13-06-2013
Note: I have edited the code. There was a little bug; because in many months, first week and last week are not complete weeks and some days of that week belongs to another month.

If you were keen to use linq (I almost always am!), you could do this:
int year = 2013;
int month = 6;
int weekOfMonth = 2;
var dates = Enumerable.Range(1, DateTime.DaysInMonth(year, month))
.Select(day => new DateTime(year, month, day))
.GroupBy(g=> g.DayOfYear/7)
.ToList();
var week = dates.Min(g => g.Key) + weekOfMonth - 1;
var result = dates.Where(g=> g.Key.Equals(week)).Select(g => g.ToList());

try this
protected void button_click(object sender, EventArgs e)
{
DateTime dt = new DateTime(2013, 6, 1);
string[] dates = getDates(dt);
}
public string[] getDates(DateTime dt)
{
string[] result = new string[7]; ;
for (int i = getDay(dt.DayOfWeek.ToString()); i < 7; i++)
{
if (i == 0)
{
break;
}
else
{
dt = dt.AddDays(1);
}
}
for (int i = 0; i < 7; i++)
{
dt = dt.AddDays(1);
result[i] = dt.ToShortDateString();
}
return result;
}
public int getDay(string day)
{
switch (day)
{
case "Monday":
return 0;
case "Tuesday":
return 1;
case "Wednesday":
return 2;
case "Thursday":
return 3;
case "Friday":
return 4;
case "Saturday":
return 5;
case "Sunday":
return 6;
default:
return 0;
}
}## Heading ##

How do I get the month number from the year and week number in c#?

As the title says, given the year and the week number, how do I get the month number?
edit: if a week crosses two months, I want the month the first day of the week is in.
edit(2): This is how I get the week number:
CultureInfo.CurrentCulture.Calendar.GetWeekOfYear(DateTime.Now, CalendarWeekRule.FirstDay, DayOfWeek.Monday);
I'm just trying to do the reverse.

If you assume that the first day of your definition of week is the same day as the 1st day of the year, then this will work:
int year = 2000;
int week = 9;
int month = new DateTime(year, 1, 1).AddDays(7 * (week - 1)).Month;
Obviously, a true answer would depend on how you define the first day of the week, and how you define how a week falls into a month when it overlaps more than one.

This is what I ended up doing:
static int GetMonth(int Year, int Week)
{
DateTime tDt = new DateTime(Year, 1, 1);
tDt.AddDays((Week - 1) * 7);
for (int i = 0; i <= 365; ++i)
{
int tWeek = CultureInfo.CurrentCulture.Calendar.GetWeekOfYear(
tDt,
CalendarWeekRule.FirstDay,
DayOfWeek.Monday);
if (tWeek == Week)
return tDt.Month;
tDt = tDt.AddDays(1);
}
return 0;
}
I would have preferred something simpler, but it works :)

Wouldn't it also depend on the day of the week?

this should be able to help
public int getMonth(int weekNum, int year)
{
DateTime Current = new DateTime(year, 1, 1);
System.DayOfWeek StartDOW = Current.DayOfWeek;
int DayOfYear = (weekNum * 7) - 6; //1st day of the week
if (StartDOW != System.DayOfWeek.Sunday) //means that last week of last year's month
{
Current = Current.AddDays(7 - (int)Current.DayOfWeek);
}
return Current.AddDays(DayOfYear).Month;
}

Another problem you could face is that most years do not start at the beginning of a week, which shifts everything.

Assumptions:
Sunday is the first day of the week.
Partial week still counts as week 1
Outputs beginning and ending month as integer array.
public int[] getMonth(int weekNum, int year)
{
DateTime StartYear = new DateTime(year, 1, 1);
System.DayOfWeek StartDOW = StartYear.DayOfWeek;
DateTime DayOfYearWeekStart = default(DateTime);
DateTime DayOfYearWeekEnd = default(DateTime);
int x = 0;
if ((StartDOW == System.DayOfWeek.Sunday)) {
DayOfYearWeekStart = StartYear.AddDays((weekNum - 1) * 7);
DayOfYearWeekEnd = DayOfYearWeekStart.AddDays(6);
} else {
for (x = 0; x <= 7; x += 1) {
if (StartYear.AddDays(x).DayOfWeek == DayOfWeek.Sunday) {
break; // TODO: might not be correct. Was : Exit For
}
}
if (weekNum == 1) {
DayOfYearWeekStart = StartYear;
DayOfYearWeekEnd = StartYear.AddDays(x - 1);
} else if (weekNum > 1) {
DayOfYearWeekStart = StartYear.AddDays(((weekNum - 2) * 7) + x);
DayOfYearWeekEnd = DayOfYearWeekStart.AddDays(6);
}
}
int[] Month = new int[2];
Month[0] = DayOfYearWeekStart.Month;
Month[1] = DayOfYearWeekEnd.Month;
return Month;
}

You cant. You need at least the day on which the 1st week starts (or when the week starts), to get an accurate answer.

You cant. A week may start in one month and end in another.

I think you're assuming that a "week" is any group of 7 sequential days. It isn't. Given Year(2008), Week(5), you could be in either January or Febuary, depending on when your "week" starts.

In .NET 3.0 and later you can use the ISOWeek-Class.
public static int MonthOfFirstDay(int year, int week)
{
return ISOWeek.ToDateTime(year, week, DayOfWeek.Monday).Month;
}
Note that the year might not fit, as the first week of a year can already start in end of December the year before. For instance the first week of 2020 started on Monday 2019-12-30.

// Calculate the week number according to ISO 8601
public static int Iso8601WeekNumber(DateTime dt)
{
return CultureInfo.CurrentCulture.Calendar.GetWeekOfYear(dt, CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday);
}
// ...
DateTime dt = DateTime.Now;
// Calculate the WeekOfMonth according to ISO 8601
int weekOfMonth = Iso8601WeekNumber(dt) - Iso8601WeekNumber(dt.AddDays(1 - dt.Day)) + 1;

How can I get the DateTime for the start of the week?

How do I find the start of the week (both Sunday and Monday) knowing just the current time in C#?
Something like:
DateTime.Now.StartWeek(Monday);

Use an extension method:
public static class DateTimeExtensions
{
public static DateTime StartOfWeek(this DateTime dt, DayOfWeek startOfWeek)
{
int diff = (7 + (dt.DayOfWeek - startOfWeek)) % 7;
return dt.AddDays(-1 * diff).Date;
}
}
Which can be used as follows:
DateTime dt = DateTime.Now.StartOfWeek(DayOfWeek.Monday);
DateTime dt = DateTime.Now.StartOfWeek(DayOfWeek.Sunday);

The quickest way I can come up with is:
var sunday = DateTime.Today.AddDays(-(int)DateTime.Today.DayOfWeek);
If you would like any other day of the week to be your start date, all you need to do is add the DayOfWeek value to the end
var monday = DateTime.Today.AddDays(-(int)DateTime.Today.DayOfWeek + (int)DayOfWeek.Monday);
var tuesday = DateTime.Today.AddDays(-(int)DateTime.Today.DayOfWeek + (int)DayOfWeek.Tuesday);

A little more verbose and culture-aware:
System.Globalization.CultureInfo ci =
System.Threading.Thread.CurrentThread.CurrentCulture;
DayOfWeek fdow = ci.DateTimeFormat.FirstDayOfWeek;
DayOfWeek today = DateTime.Now.DayOfWeek;
DateTime sow = DateTime.Now.AddDays(-(today - fdow)).Date;

Using Fluent DateTime:
var monday = DateTime.Now.Previous(DayOfWeek.Monday);
var sunday = DateTime.Now.Previous(DayOfWeek.Sunday);

Ugly but it at least gives the right dates back
With start of week set by system:
public static DateTime FirstDateInWeek(this DateTime dt)
{
while (dt.DayOfWeek != System.Threading.Thread.CurrentThread.CurrentCulture.DateTimeFormat.FirstDayOfWeek)
dt = dt.AddDays(-1);
return dt;
}
Without:
public static DateTime FirstDateInWeek(this DateTime dt, DayOfWeek weekStartDay)
{
while (dt.DayOfWeek != weekStartDay)
dt = dt.AddDays(-1);
return dt;
}

Let's combine the culture-safe answer and the extension method answer:
public static class DateTimeExtensions
{
public static DateTime StartOfWeek(this DateTime dt, DayOfWeek startOfWeek)
{
System.Globalization.CultureInfo ci = System.Threading.Thread.CurrentThread.CurrentCulture;
DayOfWeek fdow = ci.DateTimeFormat.FirstDayOfWeek;
return DateTime.Today.AddDays(-(DateTime.Today.DayOfWeek- fdow));
}
}

This would give you the preceding Sunday (I think):
DateTime t = DateTime.Now;
t -= new TimeSpan ((int) t.DayOfWeek, 0, 0, 0);

For Monday
DateTime startAtMonday = DateTime.Now.AddDays(DayOfWeek.Monday - DateTime.Now.DayOfWeek);
For Sunday
DateTime startAtSunday = DateTime.Now.AddDays(DayOfWeek.Sunday- DateTime.Now.DayOfWeek);

This may be a bit of a hack, but you can cast the .DayOfWeek property to an int (it's an enum and since its not had its underlying data type changed it defaults to int) and use that to determine the previous start of the week.
It appears the week specified in the DayOfWeek enum starts on Sunday, so if we subtract 1 from this value that'll be equal to how many days the Monday is before the current date. We also need to map the Sunday (0) to equal 7 so given 1 - 7 = -6 the Sunday will map to the previous Monday:-
DateTime now = DateTime.Now;
int dayOfWeek = (int)now.DayOfWeek;
dayOfWeek = dayOfWeek == 0 ? 7 : dayOfWeek;
DateTime startOfWeek = now.AddDays(1 - (int)now.DayOfWeek);
The code for the previous Sunday is simpler as we don't have to make this adjustment:-
DateTime now = DateTime.Now;
int dayOfWeek = (int)now.DayOfWeek;
DateTime startOfWeek = now.AddDays(-(int)now.DayOfWeek);

using System;
using System.Globalization;
namespace MySpace
{
public static class DateTimeExtention
{
// ToDo: Need to provide culturaly neutral versions.
public static DateTime GetStartOfWeek(this DateTime dt)
{
DateTime ndt = dt.Subtract(TimeSpan.FromDays((int)dt.DayOfWeek));
return new DateTime(ndt.Year, ndt.Month, ndt.Day, 0, 0, 0, 0);
}
public static DateTime GetEndOfWeek(this DateTime dt)
{
DateTime ndt = dt.GetStartOfWeek().AddDays(6);
return new DateTime(ndt.Year, ndt.Month, ndt.Day, 23, 59, 59, 999);
}
public static DateTime GetStartOfWeek(this DateTime dt, int year, int week)
{
DateTime dayInWeek = new DateTime(year, 1, 1).AddDays((week - 1) * 7);
return dayInWeek.GetStartOfWeek();
}
public static DateTime GetEndOfWeek(this DateTime dt, int year, int week)
{
DateTime dayInWeek = new DateTime(year, 1, 1).AddDays((week - 1) * 7);
return dayInWeek.GetEndOfWeek();
}
}
}

Putting it all together, with Globalization and allowing for specifying the first day of the week as part of the call we have
public static DateTime StartOfWeek ( this DateTime dt, DayOfWeek? firstDayOfWeek )
{
DayOfWeek fdow;
if ( firstDayOfWeek.HasValue )
{
fdow = firstDayOfWeek.Value;
}
else
{
System.Globalization.CultureInfo ci = System.Threading.Thread.CurrentThread.CurrentCulture;
fdow = ci.DateTimeFormat.FirstDayOfWeek;
}
int diff = dt.DayOfWeek - fdow;
if ( diff < 0 )
{
diff += 7;
}
return dt.AddDays( -1 * diff ).Date;
}

Step 1:
Create a static class
public static class TIMEE
{
public static DateTime StartOfWeek(this DateTime dt, DayOfWeek startOfWeek)
{
int diff = (7 + (dt.DayOfWeek - startOfWeek)) % 7;
return dt.AddDays(-1 * diff).Date;
}
public static DateTime EndOfWeek(this DateTime dt, DayOfWeek startOfWeek)
{
int diff = (7 - (dt.DayOfWeek - startOfWeek)) % 7;
return dt.AddDays(1 * diff).Date;
}
}
Step 2: Use this class to get both start and end day of the week
DateTime dt = TIMEE.StartOfWeek(DateTime.Now ,DayOfWeek.Monday);
DateTime dt1 = TIMEE.EndOfWeek(DateTime.Now, DayOfWeek.Sunday);

var now = System.DateTime.Now;
var result = now.AddDays(-((now.DayOfWeek - System.Threading.Thread.CurrentThread.CurrentCulture.DateTimeFormat.FirstDayOfWeek + 7) % 7)).Date;

This would give you midnight on the first Sunday of the week:
DateTime t = DateTime.Now;
t -= new TimeSpan ((int) t.DayOfWeek, t.Hour, t.Minute, t.Second);
This gives you the first Monday at midnight:
DateTime t = DateTime.Now;
t -= new TimeSpan ((int) t.DayOfWeek - 1, t.Hour, t.Minute, t.Second);

Try with this in C#. With this code you can get both the first date and last date of a given week. Here Sunday is the first day and Saturday is the last day, but you can set both days according to your culture.
DateTime firstDate = GetFirstDateOfWeek(DateTime.Parse("05/09/2012").Date, DayOfWeek.Sunday);
DateTime lastDate = GetLastDateOfWeek(DateTime.Parse("05/09/2012").Date, DayOfWeek.Saturday);
public static DateTime GetFirstDateOfWeek(DateTime dayInWeek, DayOfWeek firstDay)
{
DateTime firstDayInWeek = dayInWeek.Date;
while (firstDayInWeek.DayOfWeek != firstDay)
firstDayInWeek = firstDayInWeek.AddDays(-1);
return firstDayInWeek;
}
public static DateTime GetLastDateOfWeek(DateTime dayInWeek, DayOfWeek firstDay)
{
DateTime lastDayInWeek = dayInWeek.Date;
while (lastDayInWeek.DayOfWeek != firstDay)
lastDayInWeek = lastDayInWeek.AddDays(1);
return lastDayInWeek;
}

I tried several, but I did not solve the issue with a week starting on a Monday, resulting in giving me the coming Monday on a Sunday. So I modified it a bit and got it working with this code:
int delta = DayOfWeek.Monday - DateTime.Now.DayOfWeek;
DateTime monday = DateTime.Now.AddDays(delta == 1 ? -6 : delta);
return monday;

The same for end of the week (in style of Compile This's answer):
public static DateTime EndOfWeek(this DateTime dt)
{
int diff = 7 - (int)dt.DayOfWeek;
diff = diff == 7 ? 0 : diff;
DateTime eow = dt.AddDays(diff).Date;
return new DateTime(eow.Year, eow.Month, eow.Day, 23, 59, 59, 999) { };
}

Thanks for the examples. I needed to always use the "CurrentCulture" first day of the week and for an array I needed to know the exact Daynumber.. so here are my first extensions:
public static class DateTimeExtensions
{
//http://stackoverflow.com/questions/38039/how-can-i-get-the-datetime-for-the-start-of-the-week
//http://stackoverflow.com/questions/1788508/calculate-date-with-monday-as-dayofweek1
public static DateTime StartOfWeek(this DateTime dt)
{
//difference in days
int diff = (int)dt.DayOfWeek - (int)CultureInfo.CurrentCulture.DateTimeFormat.FirstDayOfWeek; //sunday=always0, monday=always1, etc.
//As a result we need to have day 0,1,2,3,4,5,6
if (diff < 0)
{
diff += 7;
}
return dt.AddDays(-1 * diff).Date;
}
public static int DayNoOfWeek(this DateTime dt)
{
//difference in days
int diff = (int)dt.DayOfWeek - (int)CultureInfo.CurrentCulture.DateTimeFormat.FirstDayOfWeek; //sunday=always0, monday=always1, etc.
//As a result we need to have day 0,1,2,3,4,5,6
if (diff < 0)
{
diff += 7;
}
return diff + 1; //Make it 1..7
}
}

Here is a correct solution. The following code works regardless if the first day of the week is a Monday or a Sunday or something else.
public static class DateTimeExtension
{
public static DateTime GetFirstDayOfThisWeek(this DateTime d)
{
CultureInfo ci = System.Threading.Thread.CurrentThread.CurrentCulture;
var first = (int)ci.DateTimeFormat.FirstDayOfWeek;
var current = (int)d.DayOfWeek;
var result = first <= current ?
d.AddDays(-1 * (current - first)) :
d.AddDays(first - current - 7);
return result;
}
}
class Program
{
static void Main()
{
System.Threading.Thread.CurrentThread.CurrentCulture = CultureInfo.GetCultureInfo("en-US");
Console.WriteLine("Current culture set to en-US");
RunTests();
Console.WriteLine();
System.Threading.Thread.CurrentThread.CurrentCulture = CultureInfo.GetCultureInfo("da-DK");
Console.WriteLine("Current culture set to da-DK");
RunTests();
Console.ReadLine();
}
static void RunTests()
{
Console.WriteLine("Today {1}: {0}", DateTime.Today.Date.GetFirstDayOfThisWeek(), DateTime.Today.Date.ToString("yyyy-MM-dd"));
Console.WriteLine("Saturday 2013-03-02: {0}", new DateTime(2013, 3, 2).GetFirstDayOfThisWeek());
Console.WriteLine("Sunday 2013-03-03: {0}", new DateTime(2013, 3, 3).GetFirstDayOfThisWeek());
Console.WriteLine("Monday 2013-03-04: {0}", new DateTime(2013, 3, 4).GetFirstDayOfThisWeek());
}
}

Modulo in C# works bad for -1 mod 7 (it should be 6, but C# returns -1)
so... a "one-liner" solution to this will look like this :)
private static DateTime GetFirstDayOfWeek(DateTime date)
{
return date.AddDays(
-(((int)date.DayOfWeek - 1) -
(int)Math.Floor((double)((int)date.DayOfWeek - 1) / 7) * 7));
}

I did it for Monday, but with similar logic for Sunday.
public static DateTime GetStartOfWeekDate()
{
// Get today's date
DateTime today = DateTime.Today;
// Get the value for today. DayOfWeek is an enum with 0 being Sunday, 1 Monday, etc
var todayDayOfWeek = (int)today.DayOfWeek;
var dateStartOfWeek = today;
// If today is not Monday, then get the date for Monday
if (todayDayOfWeek != 1)
{
// How many days to get back to Monday from today
var daysToStartOfWeek = (todayDayOfWeek - 1);
// Subtract from today's date the number of days to get to Monday
dateStartOfWeek = today.AddDays(-daysToStartOfWeek);
}
return dateStartOfWeek;
}

The following method should return the DateTime that you want. Pass in true for Sunday being the first day of the week, false for Monday:
private DateTime getStartOfWeek(bool useSunday)
{
DateTime now = DateTime.Now;
int dayOfWeek = (int)now.DayOfWeek;
if(!useSunday)
dayOfWeek--;
if(dayOfWeek < 0)
{// day of week is Sunday and we want to use Monday as the start of the week
// Sunday is now the seventh day of the week
dayOfWeek = 6;
}
return now.AddDays(-1 * (double)dayOfWeek);
}

You could use the excellent Umbrella library:
using nVentive.Umbrella.Extensions.Calendar;
DateTime beginning = DateTime.Now.BeginningOfWeek();
However, they do seem to have stored Monday as the first day of the week (see the property nVentive.Umbrella.Extensions.Calendar.DefaultDateTimeCalendarExtensions.WeekBeginsOn), so that previous localized solution is a bit better. Unfortunate.
Edit: looking closer at the question, it looks like Umbrella might actually work for that too:
// Or DateTime.Now.PreviousDay(DayOfWeek.Monday)
DateTime monday = DateTime.Now.PreviousMonday();
DateTime sunday = DateTime.Now.PreviousSunday();
Although it's worth noting that if you ask for the previous Monday on a Monday, it'll give you seven days back. But this is also true if you use BeginningOfWeek, which seems like a bug :(.

Following on from Compile This' answer, use the following method to obtain the date for any day of the week:
public static DateTime GetDayOfWeek(DateTime dateTime, DayOfWeek dayOfWeek)
{
var monday = dateTime.Date.AddDays((7 + (dateTime.DayOfWeek - DayOfWeek.Monday) % 7) * -1);
var diff = dayOfWeek - DayOfWeek.Monday;
if (diff == -1)
{
diff = 6;
}
return monday.AddDays(diff);
}

This will return both the beginning of the week and the end of the week dates:
private string[] GetWeekRange(DateTime dateToCheck)
{
string[] result = new string[2];
TimeSpan duration = new TimeSpan(0, 0, 0, 0); //One day
DateTime dateRangeBegin = dateToCheck;
DateTime dateRangeEnd = DateTime.Today.Add(duration);
dateRangeBegin = dateToCheck.AddDays(-(int)dateToCheck.DayOfWeek);
dateRangeEnd = dateToCheck.AddDays(6 - (int)dateToCheck.DayOfWeek);
result[0] = dateRangeBegin.Date.ToString();
result[1] = dateRangeEnd.Date.ToString();
return result;
}
I have posted the complete code for calculating the begin/end of week, month, quarter and year on my blog
ZamirsBlog

Calculating this way lets you choose which day of the week indicates the start of a new week (in the example I chose Monday).
Note that doing this calculation for a day that is a Monday will give the current Monday and not the previous one.
//Replace with whatever input date you want
DateTime inputDate = DateTime.Now;
//For this example, weeks start on Monday
int startOfWeek = (int)DayOfWeek.Monday;
//Calculate the number of days it has been since the start of the week
int daysSinceStartOfWeek = ((int)inputDate.DayOfWeek + 7 - startOfWeek) % 7;
DateTime previousStartOfWeek = inputDate.AddDays(-daysSinceStartOfWeek);

I work with a lot of schools, so correctly using Monday as the first day of the week is important here.
A lot of the most terse answers here don't work on Sunday -- we often end up returning the date of tomorrow on Sunday, which is not good for running a report on last week's activities.
Here's my solution, which returns last Monday on Sunday, and today on Monday.
// Adding 7 so remainder is always positive; Otherwise % returns -1 on Sunday.
var daysToSubtract = (7 + (int)today.DayOfWeek - (int)DayOfWeek.Monday) % 7;
var monday = today
.AddDays(-daysToSubtract)
.Date;
Remember to use a method parameter for "today" so it's unit testable!!

Here is a combination of a few of the answers. It uses an extension method that allows the culture to be passed in. If one is not passed in, the current culture is used. This will give it maximum flexibility and reuse.
/// <summary>
/// Gets the date of the first day of the week for the date.
/// </summary>
/// <param name="date">The date to be used</param>
/// <param name="cultureInfo">If none is provided, the current culture is used</param>
/// <returns>The date of the beggining of the week based on the culture specifed</returns>
public static DateTime StartOfWeek(this DateTime date, CultureInfo cultureInfo=null) =>
date.AddDays(-1 * (7 + (date.DayOfWeek - (cultureInfo ?? CultureInfo.CurrentCulture).DateTimeFormat.FirstDayOfWeek)) % 7).Date;
Example Usage:
public static void TestFirstDayOfWeekExtension() {
DateTime date = DateTime.Now;
foreach(System.Globalization.CultureInfo culture in CultureInfo.GetCultures(CultureTypes.UserCustomCulture | CultureTypes.SpecificCultures)) {
Console.WriteLine($"{culture.EnglishName}: {date.ToShortDateString()} First Day of week: {date.StartOfWeek(culture).ToShortDateString()}");
}
}

If you want Saturday or Sunday or any day of week, but not exceeding the current week (Sat-Sun), I got you covered with this piece of code.
public static DateTime GetDateInCurrentWeek(this DateTime date, DayOfWeek day)
{
var temp = date;
var limit = (int)date.DayOfWeek;
var returnDate = DateTime.MinValue;
if (date.DayOfWeek == day)
return date;
for (int i = limit; i < 6; i++)
{
temp = temp.AddDays(1);
if (day == temp.DayOfWeek)
{
returnDate = temp;
break;
}
}
if (returnDate == DateTime.MinValue)
{
for (int i = limit; i > -1; i++)
{
date = date.AddDays(-1);
if (day == date.DayOfWeek)
{
returnDate = date;
break;
}
}
}
return returnDate;
}

We like one-liners: Get the difference between the current culture's first day of week and the current day, and then subtract the number of days from the current day:
var weekStartDate = DateTime.Now.AddDays(-((int)now.DayOfWeek - (int)DateTimeFormatInfo.CurrentInfo.FirstDayOfWeek));