using System;

using System.Collections.Generic;

using System.Linq;

​

public static class Program

{

    private static string Join(this IEnumerable<char> @string, string separator = null)

        => String.Join(separator ?? String.Empty, @string);

​

    public static void Main()

    {

        var days = new[] 

        {

            "monday",

            "TUESDAY",

            "Wednesday",

            "ThUrSdAy",

            "Friday",

            "saturDAY",

            "SUNday"

        };

​

        var result = days

            .Where(day => day.Contains("a"))

            .Select(day => Char.ToLower(day[0]) + day.Skip(1).Join().ToUpper())

            .Reverse()

            .Take(2)

            .Select(dayWithA => $"[{dayWithA.Join("]+[")}]")

            .ToList();

         // Console.WriteLine(result);

    }

}

​

​

​