sql/Desktop/eew/CompanyCoreLib/Analytics.cs

namespace CompanyCoreLib
{
    public class Analytics
    {
        public List<DateTime> PopularMonths(List<DateTime> dates)
        {
            var dateTimeCounterDictionary = new Dictionary<DateTime, int>();
            int previousYear = DateTime.Now.Year - 1;

            foreach (DateTime iterDate in dates)
            {
                if (iterDate.Year == previousYear)
                {
                    // вычисляем начало месяца для текущей даты
                    var dateMonthStart = new DateTime(
                        iterDate.Year,  // год
                        iterDate.Month, // месяц
                        1);             // день

                    if (dateTimeCounterDictionary.ContainsKey(dateMonthStart))
                    {
                        dateTimeCounterDictionary[dateMonthStart]++;
                    }
                    else
                    {
                        dateTimeCounterDictionary[dateMonthStart] = 1;
                    }
                }
            }

            return dateTimeCounterDictionary
                .OrderByDescending(pair => pair.Value)
                .ThenBy(pair => pair.Key)               
                .Select(pair => pair.Key)               
                .ToList();                              
        }
    }
}