`unit` should be 'weeks'

Raise code

        else:
            interval = self.interval

        self.period = datetime.timedelta(**{self.unit: interval})
        self.next_run = datetime.datetime.now() + self.period
        if self.start_day is not None:
            if self.unit != "weeks":
                raise ScheduleValueError("`unit` should be 'weeks'")
            weekdays = (
                "monday",
                "tuesday",
                "wednesday",
                "thursday",
                "friday",
                "saturday",