# Zeller's congruence
# year is full (four-digit) year
# month is zero-origin
# day number is one-origin
def zeller(year, month, daynumber):
    newmonth = month - 2
    newyear = year
    if newmonth < 0:
        newyear = newyear - 1
        newmonth = newmonth + 12
    return ((26 * (newmonth+1) - 2) / 10 + daynumber + (newyear%100) + (newyear%100)/4 + newyear/400 - 2 * (newyear/100) + 7777) % 7