[Okay, before we get started: if you’re reading this on Monday morning, and your office isn’t cool with you perusing completely harmless pictures of people in parkas, scarves, wool hats — oh right, and their underwear — you might want to keep moving along. If you’re looking for something interesting to accompany your morning joe, Heather Goss’ astronomy post was pretty good this weekend.]

How many times does an event have to occur for it to be considered a tradition? Do we have a hard and fast rule for that? Does one have to consider how downright batty the event is in affixing the label? Well, if three is enough and the answer to that last question is yes, then count today’s annual No Pants Metro Ride as one of DCist’s favorite traditions, even if it’s not exclusively Washingtonian. (Here’s images from both 2009 and 2008‘s editions.) The chilly air wasn’t stopping these cavalier pranksters: those carrying the pantsless torch wisely chose to maintain warmth, and only strip to their skivvies once aboard the train. Smart move.

A note in the interest of journalistic integrity: DCist staff photographer Martin Locraft, always the consummate professional, kept his pants on while obtaining these images. It’s important to maintain impartiality, you know.