Well, it’s not an Advent Calendar unless it has doors to open!
For this, we use our plywood. We are going to need 24 doors, each 7.5cm x 9cm. And we’re going to need 24 little hooks to hang them on.
The simplest way of doing this is to cut your plywood into 7.5cm wide strips, using your tablesaw. I think it looks better if you cut the strips along the grain, rather than across it, but either will do. Just make sure it is consistent.
Then use your mitresaw to cut the strips at 9cm, until you have enough doors.
Plywood strips ready to be made into doors 
Piled up on the mitresaw at the 90mm mark
We then have to drill a hole in each door with a 4mm wood drill, central and 7mm from the top. That works fine for doors made of 5mm plywood. If you’re using thicker plywood, you may have to drill a slightly bigger hole.
Be careful here – when you drill into plywood, you get damage when the drill leaves the other side. Always clamp a bit of scrap wood underneath and drill into that.
And once you’ve done one hole, you can use a door as a template for the others.
What happens if you don’t drill into a scrap piece 🙁 
Using a drilled door as a template, with a scrap piece underneath 
Two doors, and a hook
Now, we have to fit the hooks to the advent calendar, and hang the doors on the hooks.
We can, of course, carefully measure up and put the hooks in exactly the right place – and that is, indeed, what I did with my first design. But I got just as good a result from the following method.
Start from the very top. Place a door centrally (by eye will be fine) and use an awl through the hole at the top of the door to mark the position of the hook. Make sure you go to the top of the hole when you stab the awl in.
Take the door away and use the awl to push a good deep starter for the hook screw. Then screw in the hook (a pair of pliers is often useful) and hang up your first door.
Marking the hole for the hook 
First door in place
Do the same for the next two, again by eye – it’ll be fine.
For the third row, start with the centre door – use the same method, and ensure that the hook is exactly between the two doors above. Then put the other two on either side.
Continue like this till all the doors are fitted.
Marking for the hook neatly between the doors above. 
Eight of the doors in place.
Finally, and optionally, trim the top of each door to give a roof effect. I just think it looks better than a flat top to each door.
Two different door designs 
All done!
And the advent calendar is now built. Congratulations!
The last job is to give it a nice finish and to number and decorate the doors.