Since it's the holiday season, it occurred to me that several of my games have holiday-themed modes when you play the games on certain holidays, in particular Rise of the Triad classic version (maybe new version as well?) and Crusader: No Regret. Since I usually can't be arsed to change my clock settings to see this stuff I'm gonna boot these games up at some point during Christmas to see what's new (I assume it'll work even in DOSBox). Anybody know what other games have such holiday modes?

Animal Crossing.

There are many seasons and events coded into the game, like winter and Halloween. You can mess with the Gamecube clock to see them, or you can wait for the proper time.

X-Wing
Tyrian
Lemmings

Stronghold (or maybe #2) had a christmas message in it, and shogun total war 2 had some weird christmas theme when i got it that ultimately made the graphics glitch out and the game became unplayable for some time!

I don't believe Lemmings had any holiday mode. Are you perhaps thinking of Xmas Lemmings, which was a separate release?

Dungeon Keeper has special messages for lots of dates around the year. I'd be surprised if it didn't also have one for Christmas. However, that's all it is. Just an onscreen message, possibly voiced by the narrator. It has no in-game influence as far as I know. Other things do, though. There are levels in the game that are only available during certain phases of the moon, for instance.

XD Yes. It may have been a separate release for you but the download I use has it all together.

I believe Phantasy Star: Online does but I forget if it's a full dress up or just holiday-themed drops.

I think it was both.

UT2004 also had a number of holiday themed mods and maps made for it. In my old gaming community, we used to play with Christmas mods in December (and in July), and Halloween mods around Halloween.

The NiGHTS Into Dreams demo has a Winter/Christmas mode.

Postal 2 has a Christmas mod called A Very Postal Christmas.

Duke 3D had an XMas add-on.

Costume Quest, Sacred 2

Killing Floor turns all of its zombie-likes into Christmas zombie-likes. The reindeer crawlers are really cute.

Cave Story +

Darwinia