The 12 Cent Calendar
Here's the finished calendar. It uses heads or tails to spell out a binary number for the day of week, month, and date (the binary system is used internally by all modern computers). In this configuration, Wednesday, December 31, 2008, is represented. If this explanation is gibberish to you, pop over to evil mad scientist laboratories for a breakdown and some tutorials on binary numbers.
Images via evil mad scientist laboratories.