No, it's got nothing to do with Bob Marley (he wasn't born on that date, nor did he die on that date), or Jimi Hendrix or Jim Morrison. Yes, it's Adolf Hitler's birthday, but that's not the reason either. There aren't 420 chemical compounds in cannabis (there are 315, according to LA Weekly). It's not a police code for weed (but it is a police code for homicide in some districts).
Here's the true story
It all started with a group of students in San Rafael High School in California in 1971. The group called themselves the "Waldos" and would regularly meet at 4:20 p.m. after classes and football practice around a statue of famous chemist Louis Pasteur. They would smoke a joint and search for a hidden patch of cannabis in the woods nearby. They never found it, or so the story goes. But their private references to "420 Louie" and later just "420' caught on, and there are old documents showing their use of the term.
The term spread because one of the Waldos was a friend of Grateful Dead bassist Phil Lesh. The Waldos started hanging out with the band, who picked up the term and gave it a national audience.
In the early 1990s, a reporter for cannabis magazine High Times was handed a flier that read: "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 the flyer. The rest is history.
"We're not political. We're jokesters."
—Steve Capper, one of the 'Waldos'
The Waldos are still good friends, and they're still amazed at how their code caught on.
"It's a phenomenon," said Steve Capper, who now works at a payroll financing company in San Francisco. "Most things die within a couple years, but this just goes on and on."
The Associated Press contributed to this report.