using System;

public class Program

{

    public static DateTime FirstDayOfLastWeek() 

    {

        var thisSunday = DateTime.Today.AddDays(-(int)DateTime.Today.DayOfWeek);

        return thisSunday.AddDays(-7);

    }

​

    public static void Main()

    {

        Console.WriteLine(String.Format("{0} => {1}", FirstDayOfLastWeek().ToString(), FirstDayOfLastWeek().AddDays(6).ToString()));

    }

}

​

​