The month of October in 2010 has five Fridays, five Saturdays, and five Sundays.



Task

 Write a program to show all months that have this same characteristic of five full weekends from the year 1900 through 2100 (Gregorian calendar). 
 Show the number of months with this property (there should be 201).
 Show at least the first and last five dates, in order.

Algorithm suggestions

 Count the number of Fridays, Saturdays, and Sundays in every month.
 Find all of the 31-day months that begin on Friday.

Extra credit

Count and/or show all of the years which do not have at least one five-weekend month (there should be 29).



Related tasks
 Day of the week
 Last Friday of each month
 Find last sunday of each month


using System;
 
namespace _5_Weekends
{
    class Program
    {
        const int FIRST_YEAR = 1900;
        const int LAST_YEAR = 2100;
        static int[] _31_MONTHS = { 1, 3, 5, 7, 8, 10, 12 }; 
 
        static void Main(string[] args)
        {
            int totalNum = 0;
            int totalNo5Weekends = 0;
 
            for (int year = FIRST_YEAR; year <= LAST_YEAR; year++)
            {
                bool has5Weekends = false;
 
                foreach (int month in _31_MONTHS)
                {
                    DateTime firstDay = new DateTime(year, month, 1);
                    if (firstDay.DayOfWeek == DayOfWeek.Friday)
                    {
                        totalNum++;
                        has5Weekends = true;
                        Console.WriteLine(firstDay.ToString("yyyy - MMMM"));
                    }
                }
 
                if (!has5Weekends) totalNo5Weekends++;
            }
            Console.WriteLine("Total 5-weekend months between {0} and {1}: {2}", FIRST_YEAR, LAST_YEAR, totalNum);
            Console.WriteLine("Total number of years with no 5-weekend months {0}", totalNo5Weekends);
        }
    }
}