Improv Everywhere started the event in New York in 2002, the Washington Post reported.
This year's organizers in Washington used Facebook to tell riders to meet at 2 p.m. at the L'Enfant Plaza metro station near the National Mall. Those who want to join in are urged to try to maintain a calm demeanor and to act like riding the subway in your underwear is the most normal thing in the world.
Participants are also being told to bring pants with them, because one of the rules is that if anyone in authority orders you to put your pants, on you are supposed to comply.
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