This silly question might not be that silly if you are in a country that relies on week numbers.
To answer this question using C# we can use the GetWeekOfYear method that’s available on Calendar class.
However, this method introduces some complexity. Apart from passing the date, it requires two additional parameters: CalendarWeekRule and DayOfWeek.
CalendarWeekRule controls when the first week of the year starts and can take one of the following values:
FirstDayFirstFullWeekFirstFourDayWeekDayOfWeek determines the first day of the week and can range from Monday to Sunday.Here’s an example to determine the week number for the last day of 2025:
var date = DateTime.Parse("2025-12-31", CultureInfo.InvariantCulture);
var weekNo =
CultureInfo.CurrentCulture.Calendar.GetWeekOfYear(date, CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday);
Console.WriteLine($"Week no for {date:d} with {CalendarWeekRule.FirstFourDayWeek}, {DayOfWeek.Monday} is {weekNo}");
// Week no for 31.12.2025 with FirstFourDayWeek, Monday is 53
but if we changed FirstFourDayWeek to FirstFullWeek we would get a different result.
var weekNo =
CultureInfo.CurrentCulture.Calendar.GetWeekOfYear(date, CalendarWeekRule.FirstFullWeek, DayOfWeek.Monday);
Console.WriteLine($"Week no for {date:d} with {CalendarWeekRule.FirstFullWeek}, {DayOfWeek.Monday} is {weekNo}");
// Week no for 31.12.2025 with FirstFullWeek, Monday is 52
In this example, the value CalendarWeekRule.FirstDay would give the same output as CalendarWeekRule.FirstFourDayWeek, but this might not always be the case.
But that’s not all - there’s also the ISOWeek class, which follows ISO rules for week number calculations. The results from ISOWeek may differ from those of the Calendar class.
Console.WriteLine($"ISO week for {date:d} is {ISOWeek.GetWeekOfYear(date)}");
// ISO week for 31.12.2025 is 1
So, the next time someone asks you to calculate a week number, be sure to ask them how they want it determined.