No pants day is an annual prank pulled by Improv Everywhere, who pull stunts like having a musical break out at a mall food court, having a jumbotron scoreboard appear at a little league game, having an Checkov impersonator do an unannounced reading at a New York B & N, and so forth. No nudity or even anything salacious, but the video on their website is hysterical.
http://improveverywhere.com/