using System;

public class Program

{

    public static void Main()

    {

        //set today to Sunday

        var today = DateTime.Now.AddDays(4);

        Console.WriteLine(today);

        //first variant return previous Monday if today = Sunday

        var diff = (7 + (today.DayOfWeek - DayOfWeek.Monday)) % 7;

        Console.WriteLine(today.AddDays(-1 * diff).Date);

        //second variant return sunday if today = Sunday

        Console.WriteLine(today.AddDays(System.Threading.Thread.CurrentThread.CurrentCulture.DateTimeFormat.FirstDayOfWeek - today.Date.DayOfWeek));

        //second variant return next Monday if today = Sunday

        Console.WriteLine(today.AddDays(DayOfWeek.Monday - today.Date.DayOfWeek));

        //second variant return next Monday if today = Sunday

        Console.WriteLine(today.AddDays(-(int)today.DayOfWeek + (int)DayOfWeek.Monday));

    }

}