source: buchla-68k/orig/DATE/WKDAY.C

/*
   =============================================================================
        wkday.c -- determine the day of the week by Zeller's Congruence
        Version 1 -- 1988-01-20 -- D.N. Lynx Crowe

        Uses the Gregorian calendar -- accurate from 1752 through 4901 A.D.

        short
        wkday(year, month, day)
        short year, month, day;

                year    1752..4901
                month   1..12           1 = Jan., 2 = Feb., ..., 12 = Dec.
                day     1..31

        Returns:

                0 - Sunday
                1 - Monday
                2 - Tuesday
                3 - Wednesday
                4 - Thursday
                5 - Friday
                6 - Saturday
   =============================================================================
*/

short
wkday(year, month, day)
register short year, month, day;
{
        register short cen, yinc;

        month -= 2;

        if (month < 1) {

                month += 12;
                --year;
        }

        cen = year / 100;
        yinc = year - (cen * 100);

        return(((((26 * month) - 2) / 10) + day + yinc + (yinc / 4)
                + (cen / 4) - (2 * cen)) % 7);
}