SEATTLE — Thursday marks marijuana culture’s high holiday, 4/20, when college students gather — at 4:20 p.m. — in clouds of smoke on campus quads and when pot shops in legal weed states thank their customers with discounts.

This year’s edition provides an occasion for pot activists to reflect on how far their movement has come, with recreational pot now allowed in eight states and the nation’s capital, as well as a changed national political climate that could threaten to slow or undermine their cause.

Here’s a look at the holiday’s history.

• WHY 4/20?

The origins of the date, and the term “420” generally, were long murky. Some claimed it referred to a police code for marijuana possession or that it arose from Bob Dylan’s “Rainy Day Women No. 12 & 35,” with its refrain of “Everybody must get stoned” — 420 being the product of 12 times 35.

But in recent years, a consensus has emerged around the most credible explanation: that it started with a group of bell-bottomed buddies from San Rafael High School in California, who called themselves “the Waldos.” A friend’s brother was afraid of getting busted for a patch of cannabis he was growing in the woods at Point Reyes, so he drew a map and gave the teens permission to harvest the crop, the story goes.

During fall 1971, at 4:20 p.m., just after classes and football practice, the group would meet up at the school’s statue of chemist Louis Pasteur, smoke a joint and head out to search for the weed patch. They never did find it, but their private lexicon — “420 Louie,” and later just “420” — would take on a life of its own.

The Waldos saved postmarked letters and other artifacts from the 1970s referencing “420,” which they now keep in a bank vault, and when the Oxford English Dictionary added the term last month it cited some of those documents as the entry’s earliest recorded uses .

• HOW DID ‘420’ SPREAD?

A brother of one of the Waldos was a close friend of Grateful Dead bassist Phil Lesh, as Lesh once confirmed in an interview with the Huffington Post. The Waldos began hanging out in the band’s circle, and the slang spread.

Fast-forward to the early 1990s: Steve Bloom, a reporter for the cannabis magazine High Times, was at a Dead show when he was handed a flier urging people to “meet at 4:20 on 4/20 for 420-ing in Marin County at the Bolinas Ridge sunset spot on Mt. Tamalpais.” High Times published it.

“It’s a phenomenon,” said one of the Waldos, Steve Capper, now 62 and a chief executive at a payroll financing company in San Francisco. “Most things die within a couple years, but this just goes on and on. It’s not like someday somebody’s going to say, ‘OK, Cannabis New Year’s is on June 23rd now.”‘

Bloom, now the editor in chief of Freedom Leaf Magazine, notes that while the Waldos came up with the term, the people who made the flier — and effectively turned 4/20 into a holiday — remain unknown.

• HOW IS IT CELEBRATED?

With weed, naturally. Some of the celebrations are bigger than others; Hippie Hill in San Francisco’s Golden Gate Park typically draws thousands. In Seattle, the organizers of the annual Hempfest event are anticipating about 250 people at a private party. Some pot shops are offering discounts or hosting block parties.

College quads and statehouse lawns are also known for drawing 4/20 celebrants, with the University of Colorado’s Boulder campus historically among the largest gatherings — though not so much since administrators started closing off the campus several years ago. Generally, 4/20 events in Colorado have dropped off significantly since the state legalized recreational use in 2012.

Some breweries make 4/20 themed beers — including SweetWater Brewing in Atlanta, whose founders attended CU-Boulder. Lagunitas Brewing in Petaluma, California, releases its “Waldos’ Special Ale” every year on 4/20 in honoring of the term’s coiners; it’s billed as “the dankest and hoppiest beer ever brewed at Lagunitas.”

• THE POLITICS

This year’s 4/20 follows successful legalization campaigns in California, Nevada, Maine and Massachusetts, which join Alaska, Colorado, Oregon and Washington as states that allow recreational marijuana. More than half the states allow medical marijuana.

But it remains illegal under federal law. Attorney General Jeff Sessions this month ordered a review of marijuana policy to see how it may conflict with the President Donald Trump’s crime-fighting agenda, and Homeland Security Secretary John Kelly recently called marijuana “a potentially dangerous gateway drug that frequently leads to the use of harder drugs.” That’s a view long held by drug warriors despite scant evidence for its validity.

Sixty percent of adults support legalizing marijuana, according to a Gallup poll last fall, and two-thirds of respondents in a Yahoo/Marist poll released this week said marijuana is safer than opioids.

Undermining regulatory schemes in legal pot states could prompt a backlash that would hasten the end of federal prohibition, said Vivian McPeak, a founder of Seattle’s Hempfest.

“We’re looking at an attorney general who wants to bring America back into the 1980s in terms of drug policy,” McPeak said. “I’m skeptical they can put the cannabis genie back into the bottle.”

• WHAT DOES IT MEAN?

McPeak says 4/20 these days is “half celebration and half call to action.”

For the Waldos, who remain close friends, it signifies above all else a good time, Capper says.

“We’re not political. We’re jokesters,” he said. “But there was a time that we can’t forget, when it was secret, furtive. … The energy of the time was more charged, more exciting in a certain way.

“I’m not saying that’s all good — it’s not good they were putting people in jail,” he added. “You wouldn’t want to go back there. Of course not.”