:
var culture = CultureInfo.CurrentCulture;
int weekNo = culture.Calendar.GetWeekOfYear(
new DateTime(YOUR_GIVEN_DATE_HERE),
currentCulture.DateTimeFormat.CalendarWeekRule,
currentCulture.DateTimeFormat.FirstDayOfWeek);
YOUR_GIVEN_DATE_HERE Datetime.Now( ) .
weekNos , .
.