It works pretty much like a real advent calendar; run your mouse inside the little rectangles on the picture to check the date the picture is for, then click to reveal the picture. Of course, pictures won't be revealed before the right day (so it's not that like a real advent calendar).
Click on the picture a second time to close it, purely for the experience of opening it again.
Many pictures are animated; all but one of these do not loop. To repeat an animation click on it to close the picture, then click again to re-open it.
Some animations are paired, and opening one will cause another already open animation to run again.
There are eight pictures for Christmas Day, which will appear at different times on the 25th. The full set can be seen by repeated clicking on Boxing Day and the rest of the month.
To completely reset the calendar and close all the squares properly, click the 'Delete cookie' below, and use F5 (or your browser's equivalent) to refresh the page.