public static void Main()
var today = DateTime.Now.AddDays(4);
Console.WriteLine(today);
var diff = (7 + (today.DayOfWeek - DayOfWeek.Monday)) % 7;
Console.WriteLine(today.AddDays(-1 * diff).Date);
Console.WriteLine(today.AddDays(System.Threading.Thread.CurrentThread.CurrentCulture.DateTimeFormat.FirstDayOfWeek - today.Date.DayOfWeek));
Console.WriteLine(today.AddDays(DayOfWeek.Monday - today.Date.DayOfWeek));
Console.WriteLine(today.AddDays(-(int)today.DayOfWeek + (int)DayOfWeek.Monday));