I came across what is probably the World's Smallest Calendar here, created by Mr. Massimiliano.
The size is so small, that it can be comfortably printed at the back of a business card.
If you like this idea, you can download the calendar for personal or non-profitable uses. If you wish to use this design for commercial purposes, please contact Mr. Massimiliano, by leaving a comment on his blog.
Download the rar file or pdf file.