Get the last weekday in a given month. e.g:

        >>> # the last monday in Jan 2013
        >>> Calendar.get_last_weekday_in_month(2013, 1, MON)
        datetime.date(2013, 1, 28)

def get_last_weekday_in_month(year, month, weekday):
        """Get the last weekday in a given month. e.g:

        >>> # the last monday in Jan 2013
        >>> Calendar.get_last_weekday_in_month(2013, 1, MON)
        datetime.date(2013, 1, 28)
        """
        day = date(year, month, monthrange(year, month)[1])
        while True:
            if day.weekday() == weekday:
                break
            day = day - timedelta(days=1)
        return day