Well, according to its Facebook invite, today was No-Pants Metro Ride 2013 - part of an annual global event started by Improv Everywhere. Participants started at Hancock Park in DC and traveling between Metro Center, Chinatown, and L'Enfant Metro Stations. The point of the day was not to "protest pants. It was just a way "to enjoy the hilarity that ensues when 400 people one day just...forget their pants."
Enjoy these great pics from the Capitol Improv Facebook page.