GetWeekOfYear varies depending on the FirstDayOfWeek and the CalendarWeekRule used. : Calendar I18N « Date Time « C# / CSharp Tutorial

1. C# / CSharp Tutorial
2. Date Time
3. Calendar I18N

//If the specified date is the last day of the year, GetWeekOfYear returns the total number of weeks in that year.

using System;
using System.Globalization;

public class SamplesCalendar  {
   public static void Main()  {
      CultureInfo myCI = new CultureInfo("en-US");
      Calendar myCal = myCI.Calendar;

      CalendarWeekRule myCWR = myCI.DateTimeFormat.CalendarWeekRule;
      DayOfWeek myFirstDOW = myCI.DateTimeFormat.FirstDayOfWeek;

      Console.WriteLine( "The CalendarWeekRule used for the en-US culture is {0}.", myCWR );
      Console.WriteLine( "The FirstDayOfWeek used for the en-US culture is {0}.", myFirstDOW );
      Console.WriteLine(myCal.GetWeekOfYear( DateTime.Now, myCWR, myFirstDOW ));

      DateTime LastDay = new System.DateTime( DateTime.Now.Year, 12, 31 );
      Console.WriteLine( "There are {0} weeks in the current year ({1}).", myCal.GetWeekOfYear( LastDay, myCWR, myFirstDOW ), LastDay.Year );

   }

}