The Simpsons road rage had christmas (and haloween i think) characters that would show up on the certain dates.

I'm pretty sure Hit and run had those themes also.

If I remember correctly, Persona 3 has some holiday/seasonal events.

Killing floor has holiday themes.

The You Don't Know Jack series of games would have unique introductions based on time of day / date / other factors... always made me smile :P

Operation: Flashpoint (now Arma Somethingoranother) gets festive right around December 25th. But that's based on the date set in Windows, and not any specific events within the game.

Thanks for all of the suggestions! I can't believe I forgot about the Harvest Moon series! I'll definitely have to play through Bully again, and Merry Gear Solid seems like fun. Also, I'm not sure if any servers are still running it, but there was a mod for HL1 called Snow War that revolved around throwing snowballs. If only they'd made a version on the Source Engine!

In terms of mods, I saw a few Halloween themed ones for Minecraft and Oblivion that I'll try out next month.