using System;

using System.Collections.Generic;

using System.Linq;                  

public class Program

{

    public static void Main()

    {

        DateTime StartDate = DateTime.Now;

            DateTime EndDate = DateTime.Now.AddDays(90);

​

            var dates =

                                Enumerable.Range(0, (int)((EndDate - StartDate).TotalDays) + 1)

                                              .Select(n => StartDate.AddDays(n))

                                              .GroupBy(a1 => a1.Month,

                                                      a2 => a2,

                                                      (baseFecha, fechas) => new

                                                       {

                                                         mes = baseFecha,

                                                         minimo = fechas.Min(),

                                                         maximo = fechas.Max()

                                                       }

                                                      );

​

          dates.ToList().ForEach(s => Console.WriteLine(s));

    }

}