Do you customize your forum around the holidays, add any Christmas things, maybe a special Christmas version of your skin etc? I will this year, seems fun to me :p
We held a Christmas banner competition for our members last year and changed the statusicons to those candy sticks on vbimageworks and also changed the buttons for new threads etc to red ones with snow on also from vbimageworks.
I usually have the forumhome decorated with the flying snowflakes with this mod .... Snow for your site :-) - vBulletin.org Forum
Depends on the forum. My two fansites, absolutely! My main forum (tvotb), not so much, because I've done a lot of work to get that custom theme where it is.
I have a winter theme that I put up in December to get everyone in the holiday mood; it isn't overtly Christmas-y though. There's a winter fairy floating around with a candy cane, but the primary symbol I use is the star, which works for just about any religion.
Usually just do a little something to that logo. Add some snow and presents. I do not change the whole theme.