There are rules: You must wear a Santa suit or facsimile thereof. A hat is not enough.
Be jolly, let random strangers sit on your lap, give out gifts and frequently bellow "ho,ho,ho!"
SantaCon -- celebrated in several cities around the country this weekend -- is one of the biggest pub crawls in the world.
The main requirement of this highly disorganized event is that revelers in red be prepared to meander en masse, inspiring (and drinking) good cheer.
Also known as Santarchy, Santa Rampage, the Red Menace and Santapalooza, it's promoted on homegrown Web sites as a "not-for-profit, non-political, non-religious and non-logical Santa Claus convention, attended for absolutely no reason."
And you better watch out! Santa, after all, is an anagram for Satan.
From Vancouver to London, the annual event is noted not only for its collection of cheap Santa suits, but for merrily bawdy behavior, including the singing of naughty Christmas carols.
"Somebody has to decide where, and we all show up," said Steve Hopson, a professional photographer who plans take part in festivities in Austin, Texas, Saturday.
"There are not real organizers," he told ABCNews.com. "The word just gets out and we all show up in red clothing."
Celebrants are not supposed to mess with children or policemen or get so inebriated they ruin it for everyone else, at least not in New York City.
"SantaCon is not a bar crawl, it's a convention," notes the Santacon Web site. "Santa does not advocate breaking open container laws! Pay your own damn bar tab and tip bartenders well for putting up with Santa."
Or more specifically: "Don't be that Santa."
"No force on earth can stop 100 Santas," proclaims a Web history of Santacon and its other iterations.
Internet legend has it that the first event was held in San Francisco in the 1970s. The founder of the city's Suicide Club organized a prank event after reading about a Danish group that mobbed a Copenhagen department store dressed as Santas.
Before then, the club was notorious for climbing suspension bridges, throwing costumed events in cemeteries and even infiltrating cult groups like the Moonies and the American Nazi Party, leaving suicide notes from the "Caliph of the North Pole."
The concept was resurrected by the city's Cacophony Society in 1994, which hoped to create a psychedelic reaction similar to public pranks that mixed guerrilla street theater and public intoxication.
Santa's Coming to Town
In the decade since, SantaCon has evolved into "just fun and totally pointless," according to its Tulsa, Okla., Facebook group. In that city in 2005, a horde of Santas on bikes clogged up rush-hour traffic.
About 200 rosy revelers turned up at Vancouver's SantaCon last weekend, crowding the sky trains and mall escalators shouting, "Merry Christmas."
Last year's Portland, Maine, SantaCon included several reindeer, a posse of elves, one pirate Santa and an advent calendar. This year, organizers predict an ecumenical theme, including menorahs and a Hanukkah chicken.
Jesse Baines, one of the Portland participants, told local press that it's all good-natured fun.
"SantaCon brings a new sense of humor and community to downtown Portland. It's a chance to meet people, take oneself a little less seriously and celebrate the season," she said.
In Japan, revelers offer Yuletide charity with Santa litter-picking outings.
In 2005 in London a SantaCon do-gooder paid off parking tickets, according to Reuters.
Drivers found this note on their windshields: "Don't let this ticket spoil your Christmas. Here's £30 to pay it off. Merry Christmas, Parking Ticket Santa."
Santas Mob Malls and Bars
In Texas this weekend, Austin photographer Hopson will be part of that city's Santa Rampage, shooting portraits of the wildly costumed participants.
The first year, in 2002, about 75 revelers turned out, but last year saw more than 500.
Hopson dresses professionally in a red vest, carrying North Pole press credentials.
"Austin, being the weird place that it is -- our folks make costumes more imaginative than in other places," he told ABCNews.com. "We have pirates, squid Santas and lots of elves."
They dress not only as Santas, but Hanukkah Harriet, reindeer, Christmas trees and fruit cakes.
"We expect a huge mob this year, mostly young adults," said Hopson.
"It's essentially nothing but a pub crawl, and we actually meet, have dinner and go to the entertainment district with hundreds of clubs," he said. "The costumes are outrageous and some are rather suggestive. You have to be out on the street and get into it."
Some are not so into it.
Last year, the event turned raucous when a rogue group protested Austin's Santarchy, attacking the jolly icon for all the "harm he'd caused," said Hopson.
Christine Malik, a 42-year-old computer support specialist who goes by the name Piper Santa because she plays the flute, participates in both Austin's tamer daylight gathering and the raucous night crawl.
During the day, Santas go to shopping malls and ride on city buses, singing holiday carols. By night, they hit the bars and restaurants downtown to spread and imbibe in Christmas cheer.
"There are hundreds of us -- enough to cause a spectacle," she told ABCNews.com.
Or turn people off.
"Some people think that Santa will be too rowdy and not let him in," said Malik.
One upscale hotel in Austin banned them from using the hotel's grand staircase for a group photo last year.
"They said, 'We can't have a mob of Santas anymore," she said. "It's a security thing."
Indeed, some Santas are naughty, not nice.
In 2005, the Auckland, New Zealand SantaCon turned into a political protest against the commercialization of Christmas. Participants started a small riot, looting stores, throwing bottles at passing cars and assaulting security guards.
At least two bystanders were injured and three were arrested, according to the Web siteAllExperts.com.
But to most participants, that type of incident flies in the face of the real meaning of SantaCon, which is bringing joy to the world.
"The best part is the reaction of people when hundreds of Santas show up somewhere," said Malik. "First you have the surprise on their faces, and then it turns into smiling and laughing. That's what we do it for."