Тип индексатора: объяснить работу кода - C#

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
 
namespace Использование__индексаторов_1
{
 
    // Using a string as an indexer value
    class DayCollection
    {
        string[] days = { "Sun", "Mon", "Tues", "Wed", "Thurs", "Fri", "Sat" };
 
        // This method finds the day or returns -1
        private int GetDay(string testDay)
        {
 
            for (int j = 0; j < days.Length; j++)
            {
                if (days[j] == testDay)
                {
                    return j;
                }
            }
 
            throw new System.ArgumentOutOfRangeException(testDay, "testDay must be in the form "Sun", "Mon", etc");
        }
 
        // The get accessor returns an integer for a given string
        public int this[string day]
        {
            get
            {
                return (GetDay(day));
            }
        }
    }
 
    class Program
    {
        static void Main(string[] args)
        {
            DayCollection week = new DayCollection();
            System.Console.WriteLine(week["Fri"]);
 
            // Raises ArgumentOutOfRangeException
            System.Console.WriteLine(week["Sun"]);
 
            // Keep the console window open in debug mode.
            System.Console.WriteLine("Press any key to exit.");
            System.Console.ReadKey();
        }
    }
}
// Output: 5