def calendar_dates(self, val):
        """
        Update ``self._calendar_dates_g``
        if ``self.calendar_dates`` changes.
        """
        self._calendar_dates = val
        if val is not None and not val.empty:
            self._calendar_dates_g = self._calendar_dates.groupby(
                ["service_id", "date"]
            )
        else:
            self._calendar_dates_g = None
Update ``self._calendar_dates_g``
        if ``self.calendar_dates`` changes.