Here's an erlang version (~586 chars):
-module (xmas).
-export ([xmas/0]).
xmas() ->
W = ["twelve drummers drumming, ",
"eleven pipers piping, ",
"ten lords a-leaping, ",
"nine ladies dancing, ",
"eight maids a-milking, ",
"seven swans a-swimming, ",
"six geese a-laying, ",
"five gold rings, ",
"four calling birds, ",
"three french hens, ",
"two turtle doves, and ",
"a partridge in a pear tree."],
io:format(lists:foldl(
fun(X,Acc) -> Acc ++ X ++ "~n" end, "",
["On the " ++ day_str(Q) ++
" day of Christmas, my true love gave me " ++
lists:foldl(
fun(X,Acc)-> Acc++X end,
"", lists:nthtail(12-Q,W)) ||
Q <- lists:seq(1,12)]),[]).
day_str(Q) ->
case Q of
1 -> "1st";
2 -> "2nd";
3 -> "3rd";
N -> erlang:integer_to_list(N,10) ++ "th"
end.