using System;

using System.Globalization;

​

​

public class Program

{

​

    public static void Main()

    {

        string s = "2018-08-17 13:26";

​

        DateTime dt = DateTime.ParseExact(s, "yyyy-MM-dd HH:mm", CultureInfo.InvariantCulture);

        bool isSunday = dt.DayOfWeek == 0;

        var dayOfweek = isSunday == false ? (int)dt.DayOfWeek : 7;

        DateTime startOfWeek = dt.AddDays(((int)(dayOfweek) * -1) + 1);

        Console.WriteLine(startOfWeek);

    }

}