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?