April 1st
I don't get into April Fool's Day. I think my nefarious side went to sleep somewhere around the time just after I saran wrapped my 12th grade area office (VP, class counselor, etc.). Since then, I can't seem to come up with good, mysterious, (harmless) and intelligent pranks. So I just don't bother.
I do, however, get a real kick out of what folks on the internets come up with.
For instance, QWANTZ (Dinosaur Comics), XKCD and Questionable Content are all currently redirecting their current comics' pages to each others' sites in a revolving "OMG WHAT HAPPEND TO TEH WORLDZ!!" sort of way that makes you seriously wonder "Did I click on the right link?".
ThinkGeek, who has done does this every year since 2001, has come up with an excellent set of not-actually-real-but-you-want-them-to-be-real goods for your perusal. I'm digging the Defendius Labyrinth Security Lock, myself.
Last year, they featured an 8-bit tie which was so popular, they actually made one.
Slashdot usually posts some funny, totally untrue, stories. I'm not saying they necessarily forgot it was April 1st, but they do seem to be taking their sweet time.