See the following snippet: Day of the week.

    ##################################################################
+#########
    #
    # day_of_week($year, $month, $day)
    #
    # Valid from 14 September 1752 onwards. $month and $day are 1 inde
+xed.
    #
    # Returns: 0 .. 6 where 0 is Sunday.
    #
    # Algorithm by Tomohiko Sakamoto from the C FAQ, section 20.31.
    #
    sub day_of_week {

        my ($year, $month, $day) = @_;

        my @offset  = (0, 3, 2, 5, 0, 3, 5, 1, 4, 6, 2, 4);
        $year      -= $month < 3;

        return ($year + int($year/4) - int($year/100) + int($year/400)
                      + $offset[$month-1] + $day) % 7;
    }
[download]

--
John.