using System;

using System.                   

public class Program

{

    public static void Main()

    {

        var EYear = 2021;

        var EndMonth = 12;

        DateTime LastDate = new DateTime(EYear, Int16.Parse(EndMonth), 1);

        string strFirstDate = string.Format("{0:yyyy/MM/dd}", FirstDate);

        string strLastDate = string.Format("{0:yyyy/MM/dd}", LastDate.AddMonths(1).AddDays(-LastDate.AddMonths(1).Day));

​

    }

}