Ole Arntzen has a website that allows you to download patterns for (and gives instructions for assembling) calendars on both rhombic and pentagonal dodecahedra.
Overall, his site is superior to this one because his calendar templates are generated by a computer script and these are static. Unfortunately, his pattern for the rhombic dodecahedron requires that it be printed on A4 paper and I didn't have any. Also the folding directions were a bit hard to follow for me, and the final object was a bit small (when I was finally finished my completed calendar was only about a centimeter bigger ;-(
This is a pretty fun project, but it may eat up most of an afternoon...
First print the following patterns (I put them in seperate files in case you want to use different colors).
Updated for 2006!
On second thought, it might be better to have a look at these directions first.