using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        List<States> states = new List<States>

            {

                new States { State = "Ak", Date= new DateTime(2014,11,27)},

                new States { State = "AL", Date= new DateTime(2014,11,27)},

                new States { State = "HI", Date= new DateTime(2014,11,30)}

            };

​

            var result = states.GroupBy(x => x.Date)

                               .Select(x => new { State = String.Join(",",x.Select(z => z.State)), Date = x.Key.ToShortDateString() });

         foreach (var item in result)

            {

                Console.WriteLine("States: {0}, Date: {1}", item.State, item.Date);

            }

    }

}

​

 public class States

        {

            public string State { get; set; }

            public DateTime Date { get; set; }

        }