I found this site (May Days of 2008) while googling, and just thought I'd mention a few:
International Fake British Accent Day – Talk with a fake British accent all day. If you are bloody British, then just talk and you're all set. Obviously, this one is good for Tim and
International Mountain Dew Day – Drink Mt. Dew, regular or diet. If you don't like it, then wear the shirt or make a statement. For
International Be Sexually Inappropriate With Your Friends Day – This is routine for most, but make every flirtatious move you make on your friends count. Do anything your heart desires on this day. Too bad this isn't on Saturday; it could be fun at Rocky Horror…
International Videogame Marathon Day – Play lots of video games all day today. Guitar Hero/Rock Band can be used again if need be. For
International Humorous T-Shirt Day – Wear a funny and clever T-shirt today. Don't have one? Buy or make one. I've got this one covered. I could probably wear a different one each hour, except that many of mine aren't safe for work (and this is a work day).