The light rail is going to be busy this weekend, especially in and around downtown Phoenix. Like, crazy busy.

After all, there will be some big concerts and a Phoenix Suns game taking place, not to mention that gigantic fan event for the College Football Playoff National Championship that will take up four whole blocks.

Oh, and there’s also the annual No Pants Light Rail Ride on Sunday, January 10, that will bring a hundred or so people into downtown. And it will be hard to miss anyone who’s participating in the event, since (as its name implies) they won’t be wearing pants of any kind while riding the rails. They’ll all have underwear on, of course, as well as other items of clothing like socks, shoes, shirts, jackets, and whatnot — just not any pants.

It's part of the M.O. of the annual event, which is also staged in other cities around the world on the same day, where participants gleefully eschew slacks, skirts, and shorts in public and hop aboard public transportation trains like the Underground in London or the New York City subway system.

It's organized every year by various local chapters of the prank collective and flash mob group Improv Everywhere and is a practical joke, social experiment, and “celebration of silliness” rolled into one.

Jeff Moriarty of Improv Arizona, which has organized the Phoenix ride since 2009, says the event is major fun, partly because it involves breaking a societal taboo of sorts by going pantsless in public. And in a more conservative place like the Valley, he says, it can be a blast to upset social norms.

“[It’s] a chance to thumb your nose, or knees, at the stodgy, conservative population of Phoenix and show them that, ‘Yes, some of us in this town aren’t so uptight,’” Moriarty says. “Some people take themselves too damned seriously, and it's really tough to do that with no pants on. And maybe to exercise a bit of freedom in a time where people are so worried about anyone acting unusual.”

It’s one of the reasons why Moriarty and others in Improv Arizona launched a Phoenix version of the event shortly after the Metro Light Rail debuted seven years ago.

“At the time people didn't do [things] like this in Phoenix. Nobody would bat an eye if it was in San Francisco or New York, but here?” he says. “I wanted to make people laugh, and let them know fun stuff could happen in Phoenix, too.”

Here’s how it works: Pantsless participants gather at starting points at a handful of stations throughout the Valley before boarding light rail trains bound for downtown. They’ll eventually make their way to the station at Central Avenue and Roosevelt Street, deboarding and walking a short distance to Angels Trumpet Ale House or another nearby bar for a post-ride celebration.

During the ride, they’ll get plenty of stares, quizzical looks, and curious questions from others onboard.

“We always stand out, but not always at first. I love it when people get on the train and don't notice until the doors close. Then they look down, maybe to check out their phone, and suddenly [the] realization hits them,” Moriarty says. “It makes the whole event that much more fun when you run into regular people during the day. Once they get over the surprise of seeing people in underwear, they'll want to know what’s going on.”

Whatever the response, participants are encouraged to act like nothing’s awry, which is also part of the event.

“The whole idea is to act like you just forgot to wear pants,” Moriarty says. “If everyone acts like it's a total accident, it drives people bonkers. Sometimes they'll even take off their own pants and join in. If you can't beat 'em, join 'em.”

And if you’re interested in joining all the pantsless shenanigans during this year’s ride on Sunday, we’ve got all the information you’ll need to know about the event contained in the following guide.

When and Where: The ride kicks off at 1 p.m. on Sunday, January 10, at four different starting points along the light rail system, including stations at Montebello and 19th avenues, Main Street and Sycamore in Mesa, and the Tempe Transit Center. Just like at last year’s event, participants can also gather at Sky Harbor Airport and board the PHX Sky Train at Terminal 4, which connects with the light rail at 44th Street.

Participants are encouraged to take the first train that arrives at their particular station after 1 p.m. and head towards downtown Phoenix. Everyone will deboard at the Central Avenue and Roosevelt Street station and head for the mid-ride party at Angels Trumpet Ale House.

Prices: It’s free to participate in the event, save for the price of a light rail fare. A single ride is $2 while an all-day pass will cost you $4. And while there won’t be a cover charge at Angels Trumpet, you’ll (naturally) have to pay for your food and drinks.

Age Limits: Anyone and everyone can participate in the No Pants ride, regardless of age, which is why you might encounter everyone from teenagers to grandparents participating. Angels Trumpet is also an all-ages establishment where children are allowed, albeit with an adult in tow. And (obviously) you have to be 21 and over in order to drink.

Parking: Both the Sycamore/Main and 19th Avenue/Montebello light rail stations offer free park-and-ride lots in close proximity. Pay lots and parking garages are available at Sky Harbor and near the Tempe Transit Center. Prices vary for both.

Weather: The Valley will get a bit of a respite from the El Niño-related rain this weekend. Skies will be partly cloudy and temperatures will be in the mid-50s. Thus, you’ll want to consider wearing a hoodie or light jacket to keep your upper half warm.

Food and Drink: Angels Trumpet offers more than 30 beers on tap and a full menu of upscale comfort food. And while most participants will head to the spot following the No Pants ride, there a variety of other eateries and drinkeries nearby (including Carly’s, DeSoto Central Market, FilmBar, Short Leash, and Chambers on First) if things get too crowded. Other Phoenix spots like the George and Dragon pub and The Lost Leaf will also welcome pantsless participants to stop by after the party at Angels Trumpet or before they head home.

What to Bring: Moriarty recommends bringing along a backpack or small bag if you need someplace to not only stash your pants but also keep your wallet, cell phone, keys, or other personal items. You also might want to have some reading material or something to watch during the ride in order to both pass the time and maintain the illusion that you’re just another rider. Oh, and don’t forget either a ride pass or your ID.

What Not to Bring: Um, pants.

What to Wear: Moriarty says that riders should choose underwear that’s comfortable to wear but doesn’t expose too much. So if you’re a dude who wants to rock boxers, make sure all the appropriate flaps are secured. Or you just might want to double up. “Boxers are comfy, but can be a little...revealing sometimes,” he says. “I suggest one pair for style, one pair underneath for safety.” And while thongs and marble bags might technically be permitted under current decency laws, it’s probably wiser to wear something a little less scandalous.

If you’re interested in sporting some colorful or snazzy-looking underwear or socks, that’s not only acceptable but a big part of the event. Some folks even get into cosplay territory, like one lass who mimicked the look of the Matt Smith version of Doctor Who, sans pants of course. Moriarty says it's fun to wear “something fun, colorful, and interesting [just] as long as it covers your jiggly bits.”

Those with more ordinary underwear shouldn't sweat it, though. “Don't worry about what you look like,” Moriarty says. “This is about fun, not fashion.”

However, don’t even consider busting out any long underwear or sleepwear for that matter for the ride.

“Wearing Long Johns is cheating,” Moriarty says. “And no PJs, either.”

Do's and Don'ts: Moriarty encourages participants to maintain the illusion that it's an ordinary day and that putting on pants was something that just slipped your mind. “Just act like you are surprised to have forgotten your pants,” he says. In other words, don’t spoil the joke.

And while the No Pants ride is ultimately inspired by silliness and he wants people to have fun, Moriarty advises participants to refrain from any disruptive or dangerous behavior while onboard a train. “Do not disrupt the light rail in any way,” he says.

Instead, he recommends interacting with other riders, especially those who aren’t participating and likely will pepper you with questions.

“Meet new people and have fun,” Moriarty says. “Running around without pants on makes a special bond between strangers.”