using System;

using System.Globalization;

​

public class Program

{

    public static void Main()

    {

        DateTime date = new DateTime(2016, 01, 3);

        // Gets the Calendar instance associated with a CultureInfo.

        // CultureInfo myCI = new CultureInfo("ar-AE");     

        // CultureInfo myCI = new CultureInfo("en-US");

        CultureInfo myCI = new CultureInfo("pt-PT");

        Calendar myCal = myCI.Calendar;

        // Gets the DTFI properties required by GetWeekOfYear.

        CalendarWeekRule myCWR = myCI.DateTimeFormat.CalendarWeekRule;

        DayOfWeek myFirstDOW = myCI.DateTimeFormat.FirstDayOfWeek;

        // Override?

        // myCWR = CalendarWeekRule.FirstFourDayWeek;

        // myFirstDOW = DayOfWeek.Sunday;

        //myCWR = CalendarWeekRule.FirstDay;

        myCWR = CalendarWeekRule.FirstFullWeek;

        myFirstDOW = DayOfWeek.Sunday;

        // Week of the month

        long weekOfWearOfDate = myCal.GetWeekOfYear(date, myCWR, myFirstDOW);

        long weekOfWearOfStartOfMonthDate = myCal.GetWeekOfYear(new DateTime(date.Year, date.Month, 1), myCWR, myFirstDOW);

        long weekOfTheMonth = 0;

        weekOfTheMonth = weekOfWearOfDate - weekOfWearOfStartOfMonthDate + 1;

        // When we have 53 weeks the week number can be 6

        // Otherwise values should be from 1 to 5

        Console.WriteLine("The week of the month for {0} is {1}.", date.ToShortDateString(), weekOfTheMonth);

        Console.WriteLine("");

        // Displays the total number of weeks in the current year.

        DateTime LastDay = new System.DateTime(date.Year, 12, 31);

        Console.WriteLine("There are {0} weeks in the current year ({1}).", myCal.GetWeekOfYear(LastDay, myCWR, myFirstDOW), LastDay.Year);

        Console.WriteLine("The week of the month for {0} is {1}.", date.ToShortDateString(), CalculateMonthWeekFirstDay(date, myFirstDOW, myCal));

    }

​

    private static int CalculateMonthWeekFirstDay(DateTime dt, DayOfWeek firstDay, Calendar cal)

    {

        int weekOfYear = cal.GetWeekOfYear(dt, CalendarWeekRule.FirstDay, firstDay);

        if (dt.Month == 1 && weekOfYear > 50) //this means that the month week belongs to december

        {

            DateTime firstDayOfDecember = new DateTime(dt.Year - 1, 12, 1);

            DateTime weekOfDecember = dt.AddDays(-7);

            return cal.GetWeekOfYear(weekOfDecember, CalendarWeekRule.FirstDay, firstDay) - cal.GetWeekOfYear(firstDayOfDecember, CalendarWeekRule.FirstDay, firstDay) + 1;

        }

        else

        {

            if (dt.Month == 1) // the exception above is just for the cases where the first days belong to the weeks of the previous year.

            { // because January is an edge case for calculations (1st day of the month can belong to the week of the previous year),

                return weekOfYear;

            }

            else

            {

                DateTime firstDayOfMonth = new DateTime(dt.Year, dt.Month, 1);

                return cal.GetWeekOfYear(dt, CalendarWeekRule.FirstDay, firstDay) - cal.GetWeekOfYear(firstDayOfMonth, CalendarWeekRule.FirstDay, firstDay) + 1;

            }

        }

    }

}