Many “420” revelers don’t know the origins of the word, but have vague recollections of once-heard tales about its origins. Some believe it’s the number of active chemicals in marijuana, others that it’s based on teatime in Holland. Some reference Bob Dylan’s legendary “Everybody must get stoned” refrain from his hit “Rainy Day Women No. 12 & 35” (12 multiplied by 35 does equal 420).

But in reality, it can all be traced back to a group of five California teens who used to hang out by a wall outside their San Rafael school—a meeting spot that inspired their nickname, “the Waldos.”

In the fall of 1971, the Waldos learned of a Coast Guard member who had planted a cannabis plant and could no longer tend to the crop. Provided with a treasure map (some say by the plant’s owner himself) supposedly leading to the abandoned product, the group would meet at the Louis Pasteur statue outside their high school at least once a week conduct a search. Their meeting time? 4:20 p.m, after practice (they were all athletes). The Waldos would pile into a car, smoke some pot and scour the nearby Point Reyes Forest for the elusive, free herb. One of the original members of the Waldos, Steve Capper, told the Huffington Post, “We would remind each other in the hallways we were supposed to meet up at 4:20. It originally started out 4:20-Louis, and we eventually dropped the Louis.”

They never did score the free bud, but perhaps they stumbled on to something more lasting? The term 420 was coined, allowing the high schoolers to discuss smoking pot without their parents or teachers knowing. But how did this ragtag team of treasure-seekers at a high school in California manage to spread their secret phrase internationally? For that, we turn to the Grateful Dead.

Members of the Waldos had open access, and many connections, to the band. Mark Gravitch’s father managed the Dead’s real estate. Dave Reddix’s older brother was good friends with Dead bassist Phil Lesh and managed a Dead sideband. Capper told the Huffington Post, “There was a place called Winterland, and we’d always be backstage running around or on stage and, of course, we’re using those phrases. When somebody passes a joint or something, ‘Hey, 420.’ So it started spreading through that community.”

The first time Steven Bloom ever heard the phrase “420” was during Christmas week at a Grateful Dead concert in Oakland, California, in 1990 while he was a reporter for “High Times.” Bloom was wandering through the congregation of hippies that would gather before Dead concerts, and a “Deadhead” handed him a flyer that said, “We are going to meet at 4:20 on 4/20 for 420-ing in Marin County at the Bolinas Ridge sunset spot on Mt. Tamalpais.” Bloom found the old flyer and sent it to Huffington Post. The flyer told the history of 420, referencing the Waldos of San Rafael. Once “High Times” latched on to the story, the magazine helped launch the word globally.

Today, the unofficial holiday is celebrated worldwide. Officials at the University of Colorado at Boulder and the University of California, Santa Cruz (two colleges that boast of having the biggest “smoke-outs”) attempted to push back on the growing popularity of the festivities among their students in 2009. They encouraged (or pleaded with) their students to not participate—to no avail.