using System;

public class Program

{

    public static void Main()

    {

        var sunday = new DateTime(2019,11,10); // This is a Sunday

        var startOfTheWeek = sunday.AddDays(DayOfWeek.Monday - sunday.DayOfWeek); 

​

        Console.WriteLine("Date: " + startOfTheWeek); // This should give Monday 4th November 2019 but gives Monday 11th November 2019

    }

}

​

​

​

​