MMO's do this often, and WoW does it the best. Fantastic events, lots and lots of fun, screwing around with the wacky gadgets you get. And they don't just celebrate Christmas and Halloween, but even the Oktoberfests in Germany.
I remember raiding cities while everyone was dressed up as a Christmas 'elf' (e.a gnome in a little suit), parading around with the presents you got from the big ol' tree. And, best of all, raiding the enemy's city to take kill their Santa Claus (or better, as a rogue, sneaking inside the city and make a hit-and-run attack on him, oh the rage). Good times, gods it´s been a year now.