第一种方法：

from datetime import datetime, timedelta
weekdays = ['Monday','Tuesday','Wednesday','Thursday',
            'Friday','Saturday','Sunday']
def get_previous_byday(dayname, start_date=None):
    if start_date is None:
        start_date = datetime.today()
    day_num = start_date.weekday()
    day_num_target = weekdays.index(dayname)
    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

print( datetime.today() )
print( get_previous_byday('Monday') )
print( get_previous_byday('Monday', datetime(2016, 8, 28)) )

 第二种方法，用dateutil模块

from datetime import datetime
from dateutil.relativedelta import relativedelta
from dateutil.rrule import *
d = datetime.now()
print(d)
print(d + relativedelta(weekday=FR))
print(d + relativedelta(weekday=FR(-1)))