From their website:

“No Pants Day is a day where everyone, be they students, respectable businessmen, or cherished community leaders, leave their pants behind. Usually this means wearing thick, appropriately modest boxer shorts, but bloomers, slips, briefs, and boxer-briefs all work as well. Things like skirts, shorts, and dresses don’t count, because people are encouraged to revel in the absence of pants, and not replace pants with other clothing. The point is to relax and enjoy the humor inherent in people not wearing pants.”

I like the fact that the endorcement section near the bottom is totally empty.