System.Globalization.GregorianCalendar.GetWeekOfYear C# (CSharp) Method

        public override int GetWeekOfYear(DateTime time, CalendarWeekRule rule, DayOfWeek firstDayOfWeek)
        {
            if ((int)firstDayOfWeek < 0 || (int)firstDayOfWeek > 6) {
                throw new ArgumentOutOfRangeException(
                    "firstDayOfWeek", String.Format(CultureInfo.CurrentCulture, Environment.GetResourceString("ArgumentOutOfRange_Range"),
                    DayOfWeek.Sunday, DayOfWeek.Saturday));
            }
            switch (rule) {
                case CalendarWeekRule.FirstDay:
                    return (GetFirstDayWeekOfYear(time, (int)firstDayOfWeek));
                case CalendarWeekRule.FirstFullWeek:
                    return (InternalGetWeekOfYearFullDays(this, time, (int)firstDayOfWeek, 7, 365));
                case CalendarWeekRule.FirstFourDayWeek:
                    return (InternalGetWeekOfYearFullDays(this, time, (int)firstDayOfWeek, 4, 365));
            }
            throw new ArgumentOutOfRangeException(
                "rule", String.Format(CultureInfo.CurrentCulture, Environment.GetResourceString("ArgumentOutOfRange_Range"),
                CalendarWeekRule.FirstDay, CalendarWeekRule.FirstFourDayWeek));

        }

        public WorkMonth(int year, int month, GermanSpecialDays specialDays, IEnumerable<ShortCut> shortCuts, float hoursPerDay)
        {
            this.year = year;
              this.month = month;
              this.hoursPerDay = hoursPerDay;
              this.Weeks = new ObservableCollection<WorkWeek>();
              this.Days = new ObservableCollection<WorkDay>();
              this.ShortCutStatistic = new ObservableCollection<KeyValuePair<string, ShortCutStatistic>>();
              // TODO which date should i take?
              this.ReloadShortcutStatistic(shortCuts);

              var cal = new GregorianCalendar();
              WorkWeek lastWeek = null;
              for (int day = 1; day <= cal.GetDaysInMonth(year, month); day++) {
            var dt = new DateTime(year, month, day);

            WorkDay wd = new WorkDay(year, month, day, specialDays);
            this.Days.Add(wd);
            var weekOfYear = cal.GetWeekOfYear(dt, CalendarWeekRule.FirstDay, DayOfWeek.Monday);
            if (lastWeek == null || lastWeek.WeekOfYear != weekOfYear) {
              lastWeek = new WorkWeek(this, weekOfYear);
              lastWeek.PropertyChanged += new PropertyChangedEventHandler(this.WeekPropertyChanged);
              this.Weeks.Add(lastWeek);
            }
            lastWeek.AddDay(wd);
              }
        }