@abc.abstractmethod<EOL><INDENT>def get_weekdays(self):<DEDENT>
return<EOL>
Gets the weekdays of the schedule.

        On a Gregorian calendar, Sunday is zero.

        :return: the weekdays
        :rtype: ``cardinal``
        :raise: ``IllegalState`` -- ``has_weekly_interval()`` is ``false``

        *compliance: mandatory -- This method must be implemented.*