using System;

public class Program

{

    public static DateTime GetMonday(DateTime date)

    {

        // get how many days into a week the date is (returns 0 for Monday)

        var dayOffset = (int)(date.DayOfWeek + 6) % 7;

        // subtract the number of days and return

        return date.AddDays(-dayOffset);

    }

​

    public static void Main()

    {

        var date = DateTime.Parse("2018-02-01");

        Console.WriteLine($"{date}");

        Console.WriteLine($"{GetMonday(date)}");

    }

}