We've tried to make it a point to fun things up for the holidays around here, with special holiday themes. Some were tres cool, others were a bit hard on the eyes.
After we came out with the daily rotating themes we seemed to drop off on creating new ones.
Since we went the way of the CSS themes, we've slowly gone back and re-created CSS versions of many of our old themes, including the holiday themes.
I think it is fun, and breaks things up a bit. Of course, we've always had several people pitching in to create the graphics, colors and so forth.