Why wait until Dec 1st to start your advent calender, mine runs from Jan 1st through till Dec 25th.
As a project it's pretty simple, buy 15 calenders which have 24 doors in a 4x6 arrangement (actually only 360 days).
Tape them all side by side and then give them an interesting paint job.
My calender looks a bit modern arty, but each colour actually represents a different month which makes it a bit easier to figure out where you should be. Being greedy I left 2 days for Dec 25th.
This and all my other project can be found on my blog http://msraynsford.blogspot.com/