Usage: python fridays_of.py <date> <num days to count back>

fridays_of.py

# Find all friday the 13th's of the last 1000 days
# python fridays_of.py 13 1000

import datetime
import sys
one_day = datetime.timedelta(days=1)

def is_fri(dt):
    return dt.weekday() == 4

today = datetime.date.today()

if __name__ == '__main__':
    date = int(sys.argv[1])
    days_back = int(sys.argv[2])

    for i in range(days_back):
        if today.day is date and is_fri(today):
            print today
        today -= one_day