code: def _cal_monday(self, day_num):
          
          if self.options['cal_monday'] or not self.options['cal_weekend']:
              day_num -= 1
              if day_num < 0:
                  day_num = 6
          return day_num
docs: Shift the day number if we're doing cal monday, or cal_weekend is
          false, since that also means we're starting on day 1