如何在类中嵌套枚举

我想弄清楚如何在类中嵌套枚举。下面的代码是如何工作的?

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
namespace Console2
{
    class Program 
    {
        class a
        {
            enum Days { Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, Saturday };
            enum Months : byte { Jan, Feb, Mar, Apr, May, Jun, Jul, Aug, Sep, Oct, Nov, Dec };   
        }
        static void Main(string[] args)
        {          
            Type weekdays = typeof(a.Days);                   
            string s = Enum.GetName(typeof(a.Days), 4);
            Console.WriteLine(s);
            Console.WriteLine("The values of the Days Enum are:");
            foreach (int i in Enum.GetValues(typeof(Days)))
            Console.WriteLine(i);
            Console.WriteLine("The names of the Days Enum are:");
            foreach (string str in Enum.GetNames(typeof(Days)))
            Console.WriteLine(str);  
        }
    }
}