University Advent Calender as your (though maybe not your) wallpaper.
Writing about web page http://www.warwick.ac.uk/adventcalendar/
For no good reason other than because the idea got stuck in my head, a script (advent wallpaper script) which grabs an image of the University advent calender, does some stuff to it, sets it as your wallpaper and then adds a launcher to the panel that when clicked shows what's 'behind' today's door with the icon for the corresponding door as it's icon. Assuming you use GNOME for your desktop environment that is. Which statistically speaking, you probably don't.
Example screengrab (to click is to make larger):
Requires ImageMagick, gnome-web-photo, curl and Epiphany (the web browser not the holiday). Call from a cron-job or login script of something to automatically update wallpaper should you feel the desire to do so.