The week number depends on 4 variables:

1. The date you're interested in,
2. The calendar you use (e.g. Gregorian, Julian, Hebrew etc.)
3. The first day of the week
4. The week you count as 1 (e.g. the week containing Jan 1, the first 4 day week etc.)

All these parameters are included in the Calendar.GetWeekOfYear method, as shown in the following code snippet:

// instantiate the calendar used in the Netherlands
Calendar calendar = new GregorianCalendar();

// get the current week for the Netherlands
calendar.GetWeekOfYear(DateTime.Now, CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday);


// and for the USA
calendar.GetWeekOfYear(DateTime.Now, CalendarWeekRule.FirstDay, DayOfWeek.Sunday);