I dont think you need to change your website, but within your website you should ALWAYS have advertisements... For instance, when someone goes to your page,y ou should have some advertisements and banners for things within your own store.. THOSE should be changed with the seasons.. this way your site is consitently the same through out the seasons, you you remind people of the upcomming holiday with your banners and graphics.
For instance, if you went to my site, you would see that there are two banners (one on each side) as advertisements within the site. There is also four large images in the center that could be used for advertisements.. Since we dont do much for seasons other than x-mas, we dont have anything special up there yet.. however when x-mas comes, we will change those to have a holiday theme
This is how the majority of websites do it.. My site has a build in banner manager where i can actually create the banners ahead of time and have them show on certain days automatically.