It's a fun exercise in UI design, trying to come up with 25 fun but fairly intuitive interactions. (I still provide instructions, but I kind of wish I didn't have to.) Also, the style of Advent Calendars, with a small distinct treat every day, makes a kind of delightful "interface". The calendar itself is basically a copy and paste of the 2009 Edition (except I made it so only one entry is visible at a time, multiple instances open tends to bog down machines.) It's HTML and CSS and Perl from when I didn't know any jQuery, and it kind of shows, though I think it gets the job done, and is fairly sophisticated at making it difficult to peek ahead.
They entries themselves are written in a Java IDE/library called processing. I love keeping my Java chops maintained a bit, and these little bite sized morels were perfect to code on the subway, to and from work. Unfortunately, Java applet support seems to get shakier and shakier as time goes on...
But Processing is a fun language especially for fun and impressive prototyping. I also use it for the 2 hour "Klik of the Month" game jams at Glorious Trainwrecks, and even for the longer 48 hour "Global Game Jams". You can see some of both at my portfolio site alienbill.com.