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

Question

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

Answer 1

var cultureInfo = CultureInfo.CurrentCulture; var calendar = cultureInfo.Calendar;

        var calendarWeekRule = cultureInfo.DateTimeFormat.CalendarWeekRule;
        var firstDayOfWeek = cultureInfo.DateTimeFormat.FirstDayOfWeek;
        var lastDayOfWeek = cultureInfo.LCID == 1033 //En-us
            ? DayOfWeek.Saturday
            : DayOfWeek.Sunday;

        var lastDayOfYear = new DateTime(date.Year, 12, 31);

         //Check if this is the last week in the year and it doesn`t occupy the whole week
        var weekNumber = calendar.GetWeekOfYear(date, calendarWeekRule, firstDayOfWeek);
        return weekNumber == 53 && lastDayOfYear.DayOfWeek != lastDayOfWeek 
               ? 1 
               : weekNumber;

It works well both for US and Russian cultures. ISO 8601 also will be correct, `cause Russian week starts at Monday.