I decided last minute to make an advent calendar for my niece and grand niece.
My birthday is at the beginning of December, and every year of my life my parents have given me an advent calendar. I have kept almost all of them, from the 80s Suzie’s Zoo when I was 6 or 7 to the German antiques I was given as an adult. I pick a couple to put out each year. I love them. They are one of my favorite things about the Christmas season. I even have one from a Cster swap wayyy back in the day.
But I have only made a couple. I thought about making a fabric one with pockets and a little character to move from day to day, but decided I will design a cut and sew one for next year. But then I saw a wooden snowflake ornament at the fabric store, and remembered the toilet paper roll trees I have seen people do, and the two things combined in my brain.
It turned out way better than I hoped. It’s made from poster board and cardstock. The cells are triangles 3" on a side, with 12 that are 4" deep and 12 that are 2" deep. Tomorrow I will write up a tutorial and get the cut files for the doors formated. But no one in my house QUITE matches my glee, so I needed to post it right away. Lol
ETA:
OK, FINALLY.
First, here is an SVG cut file
And a template for the doors, because the orientation of the tabs has been carefully planned.
If you choose to change the numbers around, for example to be orderly rather than scattered, you’ll need to actually move the numbers from door to door, rather than simply rearranging the doors themselves. Additionally, this has been carefully sized to fit exactly as directed. The tabbed triangles fit snuggly inside the cells. Minute adjustments can be made if they do not fit quite right, but even a milimeter too large or small will cause problems. If you choose to make your calendar larger or smaller, it may take some trial and error to get these resized correctly.
Materials:
Posterboard or coverstock
Decorative cardstock in at least 2 contrasting colors
Adhesive of choice (a liquid glue does warp the paper somewhat but once dry provides significant structural integrity to the piece.)
Cutting implements
I used most of 3 sheets of 22" x 28" posterboard for the main structure, plus 2 sheets of 12" x 12" glitter cardstock and 3 sheets of 8.5" x 11" foil cardstock, but I’ve formatted the cut file to print everything on 12" x 12", I just didn’t have the foil in that size.
The cut file “pages” are as follows:

A large star with diamond margins in purple, which serves to reinforce and cover the back of the calendar.
Cut lines are purple. You need one of these. 
Rows of equilateral triangles which serve as the underlayer for the doors.
Cut lines are purple. You need one of these. 
A sheet of undecorated tabbed triangles, to serve as the bottom of each cell.
Cut lines are GREEN, purple lines are SCORE lines, which you can opt to score by hand instead. You need 24 of these. Cutting twice as formatted will require two 12"x12" sheets and result in leftovers, but you can reformat to cut them from three 8.5"x11" sheets instead. 
Two different sheets of tabbed triangles with numbers and a small arc for a pulltab.
Cut lines are both red and green, purple lines are again score lines. I suggest cutting the red lines first, and green last.
In addition to the cut files you will need the following cut out of posterboard or coverstock:
12 strips 10" x 4", scored at 3", 6", and 9"
12 strips 10" x 2", scored at 3", 6", and 9"
12 strips 3.5" x 1", scored at 1", 2", and 3"
12 strips 6" x 2", scored at the halfway mark
6 strips 6" x 2", unscored
To assemble:
With each 10" long piece, fold into an equilateral triangular tube, adhering the overhang to the inside of the opposite side, matching carefully so that each cell is 3" per side. Do the same with the 3.5" x 1" pieces, adhering the excess half inch, resulting in a piece 1" deep and 1" on each of its three sides.
You should have twelve cells 4" deep and 3" on a side, twelve cells 2" deep and 3" on a side, and twelve cells 1" deep and 1" on a side.
 Starting at the center. Arrange six 4" cells into a hexagon shape and glue them together. (Each cell should be glued to two other cells, with 6 points meeting in the center.)
 Next, glue each of the other 6 4" cells to one side of the hexagon, making a Star of David shape.
 Glue one half of each of two scored 6" strips on either side of one half of an unscored 6" strip. This should result in a sort of cocked “t” shape.
 Sandwich the glued portion of this shape between the sides of two 2" cells.
 Align two 1" cells between the three loose legs of the “t” shape, centering them and assuring the points are buried as far into the crook of the T as possible.
 Continue in this manner until you have 6 such units.
 Glue one each of these units into a crook between two points of the star shape. Center the shallower units on the length of the longer units.
 Cap the bottom of each cell by gluing the tabs of the undecorated tabbed triangles to the inside of the cell. Once dry, cover the capped bottoms of the 4" cells with the decorative star shape, and each pair of 2" cells with the diamonds.
 Glue the triangles to the underside of the decorated tabbed triangles, then arrange them exactly as shown on the template. Glue the side OPPOSITE the pulltab to the inside of the associated cell.
 Add a hanger to the back of the star, near the point of the topmost 4" cell if you want to hang it, or leave it as is if you will have it flat on a table other surface.
Fill, and enjoy!