Taking a list of existing holidays, yield a list of 'shifted' days if
        the holiday falls on SUN.

def get_shifted_holidays(self, dates):
        """
        Taking a list of existing holidays, yield a list of 'shifted' days if
        the holiday falls on SUN.
        """
        for holiday, label in dates:
            if holiday.weekday() == SUN:
                yield (
                    holiday + timedelta(days=1),
                    label + ' shift'
                )