using System;
using System.Globalization;

namespace FirstDateOfWeek
{
    class Program
    {
        static void Main(string[] args)
        {
            int yr, mn, dt;
            Console.Write("\n\n Find the first day of a week against a given date :\n");
            Console.Write("--------------------------------------------------------\n");

            Console.Write(" Enter the Day : ");
            dt = Convert.ToInt32(Console.ReadLine());
            Console.Write(" Enter the Month : ");
            mn = Convert.ToInt32(Console.ReadLine());
            Console.Write(" Enter the Year : ");
            yr = Convert.ToInt32(Console.ReadLine());
            DateTime d = new DateTime(yr, mn, dt);
            Console.WriteLine(" The formatted Date is : {0}", d.ToString("dd/MM/yyyy"));
            var culture = CultureInfo.CurrentCulture;
            var diff = d.DayOfWeek - culture.DateTimeFormat.FirstDayOfWeek;
            if (diff < 0)
                diff += 7;
            d = d.AddDays(-diff).Date;
            Console.WriteLine(" The first day of the week for the above date is : {0}\n", d.ToString("dd/MM/yyyy"));
            Console.ReadKey();
        }
    }
}

public static DateTime FirstDayOfWeek(DateTime date)
{
    DayOfWeek fdow = CultureInfo.CurrentCulture.DateTimeFormat.FirstDayOfWeek;
    int offset = fdow - date.DayOfWeek;
    DateTime fdowDate = date.AddDays(offset);
    return fdowDate;
}

public static DateTime LastDayOfWeek(DateTime date)
{
    DateTime ldowDate = FirstDayOfWeek(date).AddDays(6);
    return ldowDate;
}