How can I calculate/find the week-number of a given date?

var currentCulture = CultureInfo.CurrentCulture;var weekNo = currentCulture.Calendar.GetWeekOfYear(                new DateTime(2013, 12, 31),                currentCulture.DateTimeFormat.CalendarWeekRule,                currentCulture.DateTimeFormat.FirstDayOfWeek);

Be aware that this is not ISO 8601 compatible. In Sweden we use ISO 8601 week numbers but even though the culture is set to "sv-SE", CalendarWeekRule is FirstFourDayWeek, and FirstDayOfWeek is Monday the weekNo variable will be set to 53 instead of the correct 1 in the above code.

I have only tried this with Swedish settings but I'm pretty sure that all countries (Austria, Germany, Switzerland and more) using ISO 8601 week numbers will be affected by this problem.

Peter van Ooijen and Shawn Steele has different solutions to this problem.

Here's a compact solution

private static int WeekOfYearISO8601(DateTime date){    var day = (int)CultureInfo.CurrentCulture.Calendar.GetDayOfWeek(date);    return CultureInfo.CurrentCulture.Calendar.GetWeekOfYear(date.AddDays(4 - (day == 0 ? 7 : day)), CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday);}

It's been tested for the following dates

var datesAndISO8601Weeks = new Dictionary<DateTime, int>                        {                            {new DateTime(2000, 12, 31), 52},                            {new DateTime(2001, 1, 1), 1},                            {new DateTime(2005, 1, 1), 53},                            {new DateTime(2007, 12, 31), 1},                            {new DateTime(2008, 12, 29), 1},                            {new DateTime(2010, 1, 3), 53},                            {new DateTime(2011, 12, 31), 52},                            {new DateTime(2012, 1, 1), 52},                            {new DateTime(2013, 1, 2), 1},                            {new DateTime(2013, 12, 31), 1},                        };foreach (var dateWeek in datesAndISO8601Weeks){    Debug.Assert(WeekOfYearISO8601(dateWeek.Key) == dateWeek.Value, dateWeek.Key.ToShortDateString() + " should be week number " + dateWeek.Value + " but was " + WeekOfYearISO8601(dateWeek.Key));}

  public static int GetWeekNumber(DateTime dtPassed)  {          CultureInfo ciCurr = CultureInfo.CurrentCulture;          int weekNum = ciCurr.Calendar.GetWeekOfYear(dtPassed, CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday);          return weekNum;  }

Check out GetWeekOfYear on MSDN has this example:

using System;using System.Globalization;public class SamplesCalendar  {   public static void Main()  {      // Gets the Calendar instance associated with a CultureInfo.      CultureInfo myCI = new CultureInfo("en-US");      Calendar myCal = myCI.Calendar;      // Gets the DTFI properties required by GetWeekOfYear.      CalendarWeekRule myCWR = myCI.DateTimeFormat.CalendarWeekRule;      DayOfWeek myFirstDOW = myCI.DateTimeFormat.FirstDayOfWeek;      // Displays the number of the current week relative to the beginning of the year.      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( "Therefore, the current week is Week {0} of the current year.", myCal.GetWeekOfYear( DateTime.Now, myCWR, myFirstDOW ));      // Displays the total number of weeks in the current year.      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 );   }}