This calendars papercraft is designed by ss42.com. After printing out the template on cardstock, just score all sides of each pentagon and cut every red line.
Pre-fold all scored lines.
To assemble, slide the tabs into the bottom of each notch, working your way around the center before doing the top & bottom.
If a tab does not go together well at first, just move on to the next one and then go back to the trouble spots afterward.
Once the tabs and notches are lined up they usually go together fairly easily.[via ss42]
You can download this papercraft from here: Dodecahedron Calendars & Cardstock Papercraft Free Download