If you're riding the C-Train this Sunday, don't be alarmed if a gaggle of passengers take off their pants.

They may be taking part in an unusual but long-running global event where people in cities worldwide strip off their trousers to display brightly coloured underpants, all while riding public transit.

Typically, people avert their eyes, ignoring the strange, co-ordinated disrobing, says Thomas Terashima, organizer of No Pants C-Train Ride 2019.

Others can't help but ask why.

"If people actually do ask, then we say, yes, it's part of a global event involving public transit, usually a subway," he told the Calgary Eyeopener. "Some people, they appear to be offended, because it is somewhat outside normal behaviour —but not too much."

The point isn't to break the law and promote public nudity. Instead, it's just something fun to do.

Started by Improv Everywhere in New York City in 2002, the No Pants Subway Ride movement of silliness has spread to more than 20 cities. In Vancouver and Toronto, images of colourful drawers have graced the CBC website in past years. Vancouver's SkyTrain saw more than 400 pant-less passengers one January.

Happy passengers ready to ride the rails with no pants in Vancouver. The annual event has become a celebration of bare thighs. (CBC)

In Calgary, the best they've had in 11 years has been a dozen people baring legs.

Terashima encourages new people to hop on board. They're meeting at the C-Train near the Palomino Smokehouse and Social Club on Seventh Avenue S.W. at 2 p.m. on Sunday.

Once the group has gathered, they'll board a train and get rid of those pants.

"It's a fun event — and something to do in January on a Sunday," Terashima said.

With files from the Calgary Eyeopener.