using System;

public class Program

{

    public const int DAYS_IN_A_WEEK = 7;

    public static void Main()

    {

        Console.WriteLine("Hello World");

        var firstOfNovember = new DateTime(2019, 11, 1); //Nov 1st

        Console.WriteLine($"The 4th Thursday of November is: {GetDayOfWeekInstanceOfMonth(firstOfNovember, DayOfWeek.Thursday, 4)}" );

    }

     /// <summary>

    /// A function to get the nth day of week of a month

    /// Example: Get the 2nd Monday of November 2019 returns 11/11/2019

    /// </summary>

    /// <param name="firstDayOfMonth">We use this to get the month and the year</param>

    /// <param name="dow">The DayOfWeek we're targeting</param>

    /// <param name="nthWeek">1 based index representing weeks of the month</param>

    /// <returns></returns>

    public static DateTime GetDayOfWeekInstanceOfMonth(DateTime firstDayOfMonth, DayOfWeek dow, int nthWeekIndex)

    {

      var newDate = new DateTime(firstDayOfMonth.Year, firstDayOfMonth.Month, 1);

      if (newDate.DayOfWeek != dow)

      {

        var daysOfSeperation = newDate.DayOfWeek - dow;

        var dayIncrementValue = Math.Abs(DAYS_IN_A_WEEK - daysOfSeperation) % DAYS_IN_A_WEEK;

        newDate = newDate.AddDays(dayIncrementValue);

      }

      nthWeekIndex--;

      var daysInMonth = DateTime.DaysInMonth(newDate.Year, newDate.Month);

      while (nthWeekIndex * DAYS_IN_A_WEEK + newDate.Day > daysInMonth)

      {

        nthWeekIndex--;

      }

      return newDate.AddDays(nthWeekIndex * DAYS_IN_A_WEEK);

    }

}