Got it, now to golf it. (Christmas Day is Always (Doomsday-1)). This is a great resource.

C-.

I've got it at a cool 67. :-)

my @days = qw(Sunday Monday Tuesday Wednesday Thursday Friday Saturday
+);

my $day = f($ARGV[0] || 2001);
print $days[$day], $/;

sub f {
#234567890#234567890#234567890#234567890#234567890#234567890#234567890
+#234567890
$b=1;for(1..pop){$b++;$b++if!$_%4;$b--if!$_%100;$b++if!$_%400}$b%=7
}
[download]

------
We are the carpenters and bricklayers of the Information Age.

Don't go borrowing trouble. For programmers, this means Worry only about what you need to implement.

Nice. I got 56:

sub f {
#234567890#234567890#234567890#234567890#234567890#234567890
use Time::Local;(gmtime timegm 0,0,0,25,11,-1900+pop)[6]
}
[download]

_____________________________________________________
Jeff[japhy]Pinyan: Perl, regex, and perl hacker.
s++=END;++y(;-P)}y js++=;shajsj<++y(p-q)}?print:??;

55 ;-)

sub f {
    use Time'Local;(gmtime timegm 0,0,0,25,11,-1900+pop)[6]
}
[download]

--
John.

37 chars.

However, I would say two things about this. First using ' istead of :: is so ugly that it isn't worth the 1 stroke saving. Secondly, although Perl Golf involves bending the rules as much as possible, using modules just goes against the spirit of the game.

# 39
sub xmas1 {
    use Date'Manip;Date_DayOfWeek+12,25,pop
}

# 41
sub xmas2 {
    use Date'Calc':all';Day_of_Week+pop,12,25
}
[download]

Update: I saved 2 chars by using May Day instead of Christmas. The politics of that may appeal to someone. :-)

# 37
sub xmas3 {
    use Date'Manip;Date_DayOfWeek+5,1,pop
}

# 39
sub xmas4 {
    use Date'Calc':all';Day_of_Week+pop,5,1
}
[download]

--
John.

Some simple squishing down to 49 char:

sub f {
#234567890#234567890#234567890#234567890#234567890#
$b=1;$b+=1+!$_%4-!$_%100+!$_%400for(1..pop);$b%=7
}
[download]

UPDATE - I was just compression what the parent post had, making sure my answer gave the same as that one, but not looking at the real answer. As pointed out below, '!' binds higher than '%', and thus the answer this gives is wrong. Adding parens, and the fix that japhy adds brings this to 53:

sub f {
#234567890#234567890#234567890#234567890#234567890#23
$b=1;$b+=1+!($_%4)-!($_%100)+!($_%400)for 1..pop;$b%7
}
[download]

-----------------------------------------------------
Dr. Michael K. Neylon - mneylon-pm@masemware.com || "You've left the lens cap of your mind on again, Pinky" - The Brain
"I can see my house from here!"
It's not what you know, but knowing how to find it if you don't know that's important

Sorry, Masem. I trimmed it down to 47. Hey! 47!

sub f {
#234567890#234567890#234567890#234567890#234567890#
$b=1;$b+=1+!$_%4-!$_%100+!$_%400for 1..pop;$b%7
}
[download]

_____________________________________________________
Jeff[japhy]Pinyan: Perl, regex, and perl hacker.
s++=END;++y(;-P)}y js++=;shajsj<++y(p-q)}?print:??;

What are the golfing rules on modules? I always thought modules were kind of cheating, but then I saw a couple of them in the thread here. So I thought of using Date::Christmas, but that leads to an answer well over 60 characters, but then you could really cheat and release the same module with a shorter module and function name and easily beat anything here. Which is why I always thought using modules was sort of cheating (of course if you could sneak in a 'christmasday' function into perl's internals, that'd be cheating also, but impressive :-)

Unofficially, you shouldn't use modules in perl golf unless specifically called for or given as an option in the problem. (Hypothetically, you could write a module that does the bulk of the work though still be of utility to others outside of the golf, release it to CPAN, then use that module in your golf to crush the other players). Typically golfs with 70 or more characters will be benefitted by a module use, while for golfs less than that, the module might be more trouble (as you have to add 5 + lenght(module name) automatically to use the module.

-----------------------------------------------------
Dr. Michael K. Neylon - mneylon-pm@masemware.com || "You've left the lens cap of your mind on again, Pinky" - The Brain
"I can see my house from here!"
It's not what you know, but knowing how to find it if you don't know that's important

Are you guys using another version of perl than I am? At least here, the !-operator binds more tightly than the %-operator, so that (!$_%4 &c) just won't work.

... or am I just giving away your joke now ... ?

Anyway, I guess this would work (checking the calendar). 65 chars; based on yours:

sub f {
#234567890#234567890#234567890#234567890#234567890#234567890#234567890
+#234567890
for(1..($b=pop)++){$b++;$b--if$_%4;$b++if$_%100;$b--if$_%400}$b%7
}
[download]

... or my best, so far, at 52:

sub f {
#234567890#234567890#234567890#234567890#234567890#2
$b+=$_%400?$_%100?$_%4?0:1:0:1for 1..($b=pop)++;$b%7
}
[download]

Update: Checking the calendar again ...

The Sidhekin
print "Just another Perl ${\(trickster and hacker)},"

This has a Golf Style Question attached to it...

Your answer can be trimmed down to 47 characters:

sub f {
  #23456789_123456789_123456789_123456789_123456789_12
  $b+=$_%400?$_%100?$_%4?1:2:1:2for 1..pop;$b%7+1
}
[download]

But this begs the question, should it be immediately reusable? My guess is yes, in which case this is a non-answer. The problem is that $b doesn't get reinitialized at the start of the sub so you have to either zero it or undef it in between every call.

It returns correctly the first time, but incorrectly after that; is this ok for a golf? Or should it return correctly every time?

jynx

It comes in at 66 and cheats by using a module... but it was really just so I could take Time::Piece for a spin... I *really* like it! Perhaps this snippet doesn't highlight its strong points, but you should take a look at it. I will probably be cutting way down on my Date::Manip/Date::Calc/Time::Local/POSIX::strftime usage, in favor of Time::Piece.

sub f{
#234567890#234567890#234567890#234567890#234567890#234567890#23456
use Time'Piece;(Time'Piece->strptime("25Dec".pop,"%d%b%Y"))->_wday
}
[download]

-Blake