/// <summary>
      /// Restituisce la data del lunedì del numero di settimana voluto
      /// Es. FirstDateOfWeek(2013, 5, CalendarWeekRule.FirstFourDayWeek)
      /// </summary>
      public static DateTime FirstDateOfWeek(int year, int weekNum, CalendarWeekRule rule)
      {
         DateTime jan1 = new DateTime(year, 1, 1);

         int daysOffset = DayOfWeek.Monday - jan1.DayOfWeek;
         DateTime firstMonday = jan1.AddDays(daysOffset);


         int firstWeek = Utility.WeekOfYearISO8601(firstMonday);
         if (firstWeek <= 1)
         {
            weekNum--;
         }

         return firstMonday.AddDays(weekNum * 7).Date;
      }

      public static int WeekOfYearISO8601(DateTime date)
      {
         var day = (int)CultureInfo.CurrentCulture.Calendar.GetDayOfWeek(date);
         return CultureInfo.CurrentCulture.Calendar.GetWeekOfYear(date.AddDays(4 - (day == 0 ? 7 : day)), CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday);
      }