@classmethod<EOL><INDENT>def start(cls, now, number, firstweekday=calendar.SATURDAY, **options):<DEDENT>
week = cls.mask(now, firstweekday=firstweekday, **options)<EOL>days = (number - <NUM_LIT:1>) * cls.DAYS_IN_WEEK<EOL>return week - timedelta(days=days)<EOL>
Return the starting datetime: ``number`` of weeks before ``now``.

``firstweekday`` determines when the week starts. It defaults
to Saturday.