I always thought the dodecahedron would make a perfect calendar.
Now updated for 2013.
Step 1: Required Tools
Step 2: Download Template
To draw your own pentagons:
The pentagons I used are 2 inches on a side and there are three different rectangles for the months. Months have 4, 5, or 6 rows plus one for the month name.
For the 7x7 square the side length is 2.12" and the center is 1.83" up from the bottom point of the pentagon.
For the 7x6 rectangle the size is 2.2" across, 1.9" vertical, and the center is 1.77" up from the bottom point.
And the 7x5 rectangle is 2.35" across, 1.68" vertical, and the center is 1.69" up from the bottom point.
These measurements will get the rectangle centered in the pentagon with the corners reaching the sides. To make a larger or smaller calendar I'd recommend dividing these measurements by 2 and multiplying that by the length of the side of your pentagon.
The tabs measure about 1/2 inch in from each corner and are 1/3 inch wide. The lines in the tab divide the tab in 3.
I had to use a CAD program, but more talented people could likely draw this out.
See my other instructable, Drawing a Pentagon.