When you decide to theme your site for the holidays, It doesn't have to be a drastic change, it could be something as simple as changing your CSS styles to make the colors of your site seem more christmasy. However I would strongly suggest changing your logo to fit the holidays, That is a big symbol on your page and may be all you need to do to decorate.
Google does it every year and has actually gained a reputation for it. The guy who designs them even has his own title for what the logos are called, Google Doodles. Here are some of the logos he's created. You don't have to be a pro like him, but at-least put a little effort into it like maybe put a snowflake in your logo, put a santa hat and scarf around your mascot, redesign your logo using Christmas Lights.
On the sites that I themed for the holidays I saw a 20% increase in usage of the sites starting after I did it.
I'm planning on theming my blog for the holidays as well, however the idea I have will require time.
One reader posted his site as a comment to my last decorating your site for the holidays post, he simply changed the colors of his site to fit the season.