Well,
You really don't need to write much code or do calculation for that. You can use
System.Globalization.CultureInfo.Calendar object. It has GetWeekOfYear method.
System.Globalization.CultureInfo cult_info = System.Globalization.CultureInfo.CreateSpecificCulture("no");
System.Globalization.Calendar cal = cult_info.Calendar;
int weekCount = cal.GetWeekOfYear(date, cult_info.DateTimeFormat.CalendarWeekRule, cult_info.DateTimeFormat.FirstDayOfWeek);
Check
http://msdn.microsoft.com/en-us/library/system.globalization.calendar.getweekofyear.aspx[
^]
Hope that helps. If it does then mark the answer as solution/upvote.
Thanks
Milind