Appropriately, there's no truly concrete proof of 420's history, as it's 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 friends slowly started a new routine. During the school day, they'd utter their secret phase, "420 Louis," indicating that they'd be gathering near the school's statue of Louis Pasteur to light up and celebrate the end of yet another school day.
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 marijuana patch 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.