420's history has been lost in the fog of time and more than a little jumbled by its what-were-we-just-talking-about oral traditions. However, the most repeated story traces back to San Rafael High School, which is located just north of San Francisco.
There, in the early '70s, a group of five students slowly started a new routine. During the school day, they'd utter their code word, "420 Louis," indicating that they'd be gathering near the school's statue of Louis Pasteur to collectively smoke pot and celebrate the end of yet another school day. 4:20 was the chosen time because school got out at 3 p.m. and many of the group had after-school sports activities that lasted until 4 p.m., according to a HuffPost article by Dave Reddix, one of the original group of marijuana smokers. That gave everybody enough time to meet by the statue.
The five friends became known as the "Waldos" in reference to the wall where they hung out. One of their first adventures, which they called "safaris," revolved around an abandoned cannabis crop near Point Reyes — they were hellbent on finding the secret plot so that they could score some free bud. Alas, their stoned quests came up empty-handed, but you can bet that they had plenty of fun along the way.
Little did they know their secret catchphrase would become permanently intertwined with the counterculture. At the time, The Grateful Dead was becoming more and more popular in the Marin County area and at least two of the "Waldos" had friend-of-a-friend connections to the band. It's easy to imagine how an oft-repeated secret code might begin proliferating through the raucous clamor of concerts and backstage gatherings and find favor with marijuana enthusiasts.
Throughout the years, other groups have tried to take credit for giving rise to marijuana's most famous "secret" slang. The Waldos, however, are the only ones who have letters and notes from the '70s that reference their inside jokes, including the ubiquitous 420. Once journalists for the cannabis culture magazine High Times got their resin-coated fingers on the term, it became a staple of their writing, helping propel 420 to everlasting fame.