<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _cal_monday(self, day_num):
 """Shift the day number if we're doing cal monday, or cal_weekend is false, since that also means we're starting on day 1 """

        if self.options['cal_monday'] or not self.options['cal_weekend']:
            day_num -= 1
            if day_num < 0:
                day_num = 6
        return day_num