code: def _get_offset_day(self, other):
          
          dim = ccalendar.get_days_in_month(other.year, other.month)
          mend = datetime(other.year, other.month, dim)
          wday = mend.weekday()
          shift_days = (wday - self.weekday) % 7
          return dim - shift_days
docs: Find the day in the same month as other that has the same
          weekday as self.weekday and is the last such day in the month.
  
          Parameters
          ----------
          other: datetime
  
          Returns
          -------
          day: int