using System;

public class Program

{

    public static void Main()

    {

        var meiDonNow = new DateTime(2023,5,21);

        var daysUntilNextMonday = (meiDonNow.DayOfWeek == DayOfWeek.Monday) ? 7 : ((int)DayOfWeek.Monday - (int)meiDonNow.DayOfWeek + 7) % 7;

        var nextCalculateDate = meiDonNow.AddDays(daysUntilNextMonday).Date;

        Console.WriteLine(nextCalculateDate);

    }

}

​

​