Define default roll function to be called in apply method.

def cbday_roll(self):
        """
        Define default roll function to be called in apply method.
        """
        cbday = CustomBusinessDay(n=self.n, normalize=False, **self.kwds)

        if self._prefix.endswith('S'):
            # MonthBegin
            roll_func = cbday.rollforward
        else:
            # MonthEnd
            roll_func = cbday.rollback
        return roll_func