from datetime import datetime, timedelta

if __name__ == '__main__':
    weekdays = ['Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday', 'Sunday']

    def get_previous_by_day(day_name, start_date=None):
        if start_date is None:
            start_date = datetime.today()

        day_num = start_date.weekday()
        day_num_target = weekdays.index(day_name)

        days_ago = (7 + day_num - day_num_target) % 7

        if days_ago == 0:
            days_ago = 7
        target_date = start_date - timedelta(days=days_ago)
        return target_date

    get_previous_by_day("Friday")

    # 但是如果要经常这么干，请使用dateutil包
    d = datetime.now()
    from dateutil.relativedelta import relativedelta
    from dateutil.rrule import *

    # 找最近的周五，FR是rrule里的函数
    print(d + relativedelta(weekday=FR))

    # 找上一个周五
    print(d + relativedelta(weekday=FR(-1)))