For this year's calendar, I decided to make two!
There is a relatively simple calendar on two cubes, and the somewhat more complicated calendar on a rhombic dodecahedron. Both calendars are made by plaiting strips of paper together.
The "two cubes" calendar is made from templates printed on two sheets of legal-size paper, one for the Winter/Spring months and another for the Summer/Autumn months. The second cube is just slightly smaller than the first, so they can be assembled with the second half of the year hidden inside the first (or you can thread them on a string as in the image above).
The calendar on a rhombic dodecahedron is just an update of last year's model. The template must also be printed on legal-size paper.
Have fun!