using System;

public class Program

{

    public static void Main()

    {

        TimeSpan tsStart = new TimeSpan(2,0,0);

        TimeSpan tsEnd = new TimeSpan(3,0,0);

        var startDate = CreateDate(2020, 9, 5, DayOfWeek.Sunday, tsStart);

        var endDate = CreateDate(2020,4,1, DayOfWeek.Sunday, tsEnd);        

        TimeSpan tsStartBR = new TimeSpan(23,59,59);

        TimeSpan tsEndBR = new TimeSpan(23,59,59);

        var startDateBR = CreateDate(2018, 11, 1, DayOfWeek.Saturday, tsStartBR);

        var endDateBR = CreateDate(2018, 2,3, DayOfWeek.Saturday, tsEndBR);     

        Console.WriteLine(startDate.ToString());

        Console.WriteLine(endDate.ToString());

        Console.WriteLine(startDateBR.ToString());

        Console.WriteLine(endDateBR.ToString());

    }

     private static DateTime CreateDate(int year, byte month, byte weekOfMonth, DayOfWeek originalDayOfWeek, TimeSpan timeOfDay)

        {

            var dayOfWeek = originalDayOfWeek;

​

            DateTime dt = new DateTime(year, month, 1, timeOfDay.Hours, timeOfDay.Minutes, timeOfDay.Seconds);

​

            var lastDayMonth = dt.AddMonths(1).AddDays(-1);

​

            var numbersOfWeek = 1;

​

            while (dt.Date <= lastDayMonth.Date)

            {

                if(dt.DayOfWeek == DayOfWeek.Sunday && dt.Day > 1 && weekOfMonth > numbersOfWeek)

                    numbersOfWeek += 1;   

                if (dayOfWeek == dt.DayOfWeek && weekOfMonth == numbersOfWeek)

                {

                    break;

                }            

​

                dt = dt.AddDays(1);

            }

​

            if (dt.Month != month)

                throw new Exception("The week of the month informed doesn't have the day of week informed."); //"The week of the month informed doesn't have the day of week informed."

​

            return dt;

        }

}