Perl。車輪の再発明。でも、こういうのは既存のものをあちこち探すより、
作った方が早いかも。

# RETURN VALUES
# the text name of the day of the week. Mon, Tue, Wed, ...
# EXAMPLES
# print get_day_of_week(1, 12, 2002), "\n";
# print get_day_of_week("2002-12-01"), "\n";
use Time::Local;
sub get_day_of_week {
    my ($d, $m, $y) = @_;
    ($y, $m, $d) = split("-", $d) unless (defined $m);
    my $WEEKDAY = (localtime timelocal(0, 0, 0, $d, $m - 1, $y))[6];
    return ("Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat")[$WEEKDAY];
}