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);

ANS 

Using an extension method. They're the answer to everything you know! ;) public static class DateTimeExtensions {     public static DateTime StartOfWeek(this DateTime dt, DayOfWeek startOfWeek)     {         int diff = dt.DayOfWeek - startOfWeek;         if (diff < 0)         {             diff += 7;         }         return dt.AddDays(-1 * diff).Date;     } }   Which is used thusly: DateTime dt = DateTime.Now.StartOfWeek(DayOfWeek.Monday); DateTime dt = DateTime.Now.StartOfWeek(DayOfWeek.Sunday);