Get reference dates for the holiday.

        Return reference dates for the holiday also returning the year
        prior to the start_date and year following the end_date.  This ensures
        that any offsets to be applied will yield the holidays within
        the passed in dates.

def _reference_dates(self, start_date, end_date):
        """
        Get reference dates for the holiday.

        Return reference dates for the holiday also returning the year
        prior to the start_date and year following the end_date.  This ensures
        that any offsets to be applied will yield the holidays within
        the passed in dates.
        """
        if self.start_date is not None:
            start_date = self.start_date.tz_localize(start_date.tz)

        if self.end_date is not None:
            end_date = self.end_date.tz_localize(start_date.tz)

        year_offset = DateOffset(years=1)
        reference_start_date = Timestamp(
            datetime(start_date.year - 1, self.month, self.day))

        reference_end_date = Timestamp(
            datetime(end_date.year + 1, self.month, self.day))
        # Don't process unnecessary holidays
        dates = date_range(start=reference_start_date,
                           end=reference_end_date,
                           freq=year_offset, tz=start_date.tz)

        return dates