What day of week is Halloween?

halloween.py

#!/usr/bin/env python
from __future__ import print_function
import sys
from datetime import datetime

# Usage: ./halloween.py 100 10 31
start_year = datetime.now().year
end_year = start_year + int(sys.argv[1])
month = int(sys.argv[2])
day = int(sys.argv[3])

prev = {}
for year in range(start_year, end_year):
    dow = datetime(year, month, day).strftime("%A")
    last = "(+%s)" % (year - prev[dow]) if dow in prev else ""
    print(year, dow, last)
    prev[dow] = year