GregorianCalendar Sınıfı

Constructor
Şöyle kurarız.

GregorianCalendar cal = new GregorianCalendar(GregorianCalendarTypes.Localized);

GetWeekOfYear metodu
Şöyle yaparız.

DateTime dateTime = ...;
int week = new GregorianCalendar(...)
  .GetWeekOfYear(dateTime, 
CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Saturday);