I always thought the dodecahedron would make a perfect calendar.
Now updated for 2013.

Step 1: Required Tools

To make the calendar you will need a scissors, X-acto knife or similar, tweezers, and a toothpick.

Step 2: Download Template

Download the template PDF from the link below the image preview box. Print and cut out the twelve months. Plain paper will work fine but a heavier, bonded paper will stay flat better.

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.

Step 3: First Fold

Fold the trapezoid shapes back using the edge of a table or a metal ruler.

Step 4: Tabs and Slots

Each trapezoid shape can be either a tab or a slot depending on which you need.

Step 5: Cutting Slots and Tabs

To make the slot, cut line B between lines C with the X-acto knife. To make a tab, cut both lines A and then fold the wings back behind to make a squarish tab. Check the second image in this step for a hint on cutting the tabs.

Step 6: Begin Assembly

When I assemble the calendar I make slots in the pentagon sides that I'm not yet connecting to another pentagon and put tabs in the pentagon sides that I'm about to attach to the assembly. So on the first pentagon I put slots on all five sides. On the second pentagon I make one tab and four slots. On the next three pentagons I make two tabs and three slots. And for the last pentagon I cut tabs on all five sides. Don't get ahead of yourself on the tabs and slots. Cut out the tabs and slots on each pentagon as you need them just prior to assembling it onto the dodecahedron.
Don't get too hung up on what "I" do this and "I" do that. If you can imagine a better way to keep the tabs and slots straight, go for it.

Step 7: Unfolding the Tabs

Insert each folded tab into a slot such that the trapeziod shapes are on the inside of the dodecahedron. After putting each tab into the slot, unfold the tab so the connection is permanent.

Step 8: Partially Complete View

Keep adding pentagons around the edge of the first pentagon. Remember to cut out slots and tabs on ALL FIVE edges of each pentagon as you are about to attach it to the assembly. Cutting out tabs and slots on a pentagon already assembled onto the dodecahedron is nearly impossible. Use the tweezers to insert the folded tabs into the slots as you run out of room for your fingers.

Step 9: Unfolding Final Tabs

On the last pentagon the tweezers will be a must. You will also need to use a toothpick to reach inside the dodecahedron to unfold the tabs. Unfolding the last tab may be optional as the dodecahedron should hold itself together by now.