using System;

public class Program

{

    public static void Main()

    {

        var dateFrom = new DateTime(2020,7,1);

        var start = new DateTime(dateFrom.Year, 1 + 6 * (dateFrom.Month / 7),1);

        var dateTo = new DateTime(2020,12,31);

        var end = new DateTime( dateTo.Year + dateTo.Month / 7, 7 - 6 * (dateTo.Month / 7), 1).AddDays(-1.0);

        Console.WriteLine(start);

        Console.WriteLine(end);

        var dt = new DateTime(2020,1,13);

        int month = dt.Month;

        month = ((month + 2) / 3) * 3;

        Console.WriteLine(new DateTime(dt.Year, month, 1).AddMonths(1).AddDays(-1));

    }

}