using System;

using System.Collections.Generic;

public class Program

{

    private static readonly HashSet<DateTime> Holidays = new HashSet<DateTime>();

​

    private static bool IsHoliday(DateTime date)

    {

        return Holidays.Contains(date);

    }

​

    private static bool IsWeekend(DateTime date)

    {

        return date.DayOfWeek == DayOfWeek.Sunday;

    }

​

    private static DateTime GetNextWorkingDay(DateTime date)

    {

        do

        {

            date = date.AddDays(1);

        } while (IsHoliday(date) || IsWeekend(date));

        return date;

    }

    public static void Main()

    {

        Holidays.Add(new DateTime(DateTime.Now.Year, 6, 20));

        Holidays.Add(new DateTime(DateTime.Now.Year, 6, 22));

        Holidays.Add(new DateTime(DateTime.Now.Year, 6, 23));

        Holidays.Add(new DateTime(DateTime.Now.Year, 3, 10));

        Holidays.Add(new DateTime(DateTime.Now.Year, 12, 25));

​

        var dt = GetNextWorkingDay(DateTime.Parse(@"2020-06-19"));

​

        Console.WriteLine(dt);

​

    }

}