Mark your calendar: The 9th Annual “No Pants Metro Ride” will run through Santa Monica this Sunday afternoon.

The large party of pants-less participants will meet up at 1 p.m. at Union Station to buy TAP cards and pack-up their slacks before hopping a Red Line train to downtown and then transfer to the Expo Line. The ride will go one regardless of chilly or rainy weather, although right now participants are in for a pleasant, if not breezy, forecast with highs in the 70s.

Once they make their way through 17 stops, the pants-less riders are expected to start arriving in waves at the Downtown Santa Monica station at 2:25 p.m. and then head to Tongva Park.

“The only reason is to put a smile on someone’s face,” explained organizer Leo Gonzalez. “To make their day just a little different – for participants and observers.”

There are no rules to participate, but if anyone asks you why you’re not wearing pants you are encourage to say “I forgot them” and act as if nothing is out of the ordinary. Although, Gonzalez says sometimes other riders can get flustered when no one will explain why hundreds of people forgot their pants.

“Only once was someone frustrated with me,” Gonzalez said, “but that’s because I wasn’t giving a straight answer for why I wasn’t wearing pants one of the years I wore a priest’s collar.”

The annual event started with just seven guys in New York City more than ten years ago, according to the comedy group Improv Everywhere. No matter how cold, every January the riders forget their pants, although they may don winter coats, hats, scarves and gloves. Thousands of people participate on the New York metro system. The event came to Los Angeles in 2009, and Gonzalez has been organizing it since 2010.

Last year, more than 300 people participated in the Los Angeles ride and so far about 400 have expressed an interest in doing it this year on Facebook.

Metro spokeswoman Joni Goheen says they are aware of the event and says participants should follow a few rules to avoid getting into trouble with security.

“People can’t have any private parts showing and then can’t be nude and they have to Tap,” Goheen said.

The event’s website also warns participants: “the type of underwear you want to wear will be left to your discretion, but we ask you to exercise a little common sense. The amount of skin you want to expose is your choice, but know that any citation for indecent exposure is a risk you take on your own.”

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