if (this._periodicity === 'month_week_sat')
          firstWeekLength = 7 - ((firstDayOfMonth + 1) % 7);
        else if (this._periodicity === 'month_week_sun') firstWeekLength = 7 - firstDayOfMonth;
        // 1 if month start on saturday, 2 if friday, 7 if sunday
        else firstWeekLength = 7 - ((firstDayOfMonth - 1 + 7) % 7);