The term "420" has a long association with cannabis, but until relatively recently, its origin story was lost in the mists of time. But then, just a few years ago, High Times broke the story about how and why 420 came to be. Thanks to their reporting, we now know that in 1971, a group of California teens coined the term.
They called themselves "The Waldos" for their tendency to hang out next to a wall at their San Rafael school. As the legend has it, they were on a quest to find a hidden cannabis grow, supposedly stashed somewhere in their town. The Waldos would meet weekly after sports practice at 4:20 pm; thus, this time stamp became a secret code for their mission.
But, eventually, as their quest for cannabis came up empty-handed, 420 evolved into a secret code for passing around a blunt or two. Still, this doesn't explain how a joke shared among friends became a worldwide phenomenon.
One of The Waldos' older brothers just happened to be close friends with Grateful Dead bassist Phil Lesh. In the early 1970s, the Waldos spent a fair amount of time in the vicinity of the Dead. The 420 terminology quickly spread backstage, then onstage, and eventually into the Grateful Dead fandom.