CompanyCoreLib/CompanyCoreLib/Analytics.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace CompanyCoreLib
{
    public class Analytics
    {
        public List<DateTime> PopularMonths(List<DateTime> dates)
        {
            List<DataOfOrder> dataOfOrders = new List<DataOfOrder>();
            foreach (DateTime item in dates)
            {
                var data = dataOfOrders.FirstOrDefault(x => x.Date.Month == item.Month && x.Date.Year == item.Year);
                if (data is null) dataOfOrders.Add(new DataOfOrder { Date = new DateTime(item.Year, item.Month, 1), CountOrder = 1 });
                else dataOfOrders.FirstOrDefault(x => x.Date.Month == item.Month && x.Date.Year == item.Year).CountOrder += 1;
            }
            dataOfOrders = dataOfOrders.OrderByDescending(x => x.CountOrder).ThenBy(x => x.Date).ToList();
            List<DateTime> SortList = new List<DateTime>();
            foreach (var item in dataOfOrders)
            {
                SortList.Add(item.Date);
            }
            return SortList;
        }
    }

    class DataOfOrder
    {
        public DateTime Date { get; set; }
        public int CountOrder { get; set; }
    }
}