If you want to determine the datetime of the first day of the week, you can use the following code:

// Get the first day of the week
DayOfWeek firstDay = System.Threading.Thread.CurrentThread.CurrentCulture.DateTimeFormat.FirstDayOfWeek; 
DateTime firstDayOfWeekDateTime = DateTime.Now;
while (firstDayOfWeekDateTime.DayOfWeek != firstDay)
{
    firstDayOfWeekDateTime = firstDayOfWeekDateTime.AddDays(-1);
}