using System;

​

namespace DayName

{

    public interface IDayName

    {

        string Now { get; }

    }

​

    public class DayName : IDayName

    {

        public string Now

        {

            get

            {

                return

                    DateTime.Now.DayOfWeek switch

                    {

                        DayOfWeek.Sunday => "SU",

                        DayOfWeek.Monday => "MO",

                        DayOfWeek.Tuesday => "TU",

                        DayOfWeek.Friday => "FR",

                        DayOfWeek.Saturday => "SA",

                        _ => throw new InvalidOperationException()

                    };

            }

        }

    }

​

    class Program

    {

        static void Main(string[] args)

        {

            IDayName dayName = new DayName();

            Console.WriteLine(dayName.Now);

        }

    }

}