using System.Collections.Generic;
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()
.Where(day => day.Contains("a"))
.Select(day => Char.ToLower(day[0]) + day.Skip(1).Join().ToUpper())
.Select(dayWithA => $"[{dayWithA.Join("]+[")}]")