Friday the 13th

#!/usr/bin/python3
from math import floor


def zeller(year, month, day):
    if month < 3:
        year -= 1
        month += 12
    w = year + floor(year / 4) - floor(year / 100) + floor(
        year / 400) + floor(2.6 * month + 1.6) + day
    return w % 7


print(
    len([(y, m) for y in range(2000, 3000) for m in range(1, 13)
         if zeller(y, m, 13) == 5]))