The true birth of 420 dates back to the early 1970s, when it became the hour of cannabis consumption among high school students in San Rafael. Even in mellow Marin County, stronghold of the Grateful Dead, no concessions were made to allow puffing during school hours.

So a group of stoners calling themselves "the Waldos" — because they liked to hang out in front of a wall — would pass each other in the halls, exchanging knowing glances and muttering "420 Louis!" One told the San Francisco Chronicle in 2000, "It was just a joke, but it came to mean all kinds of things, like, 'Do you have any?' or 'Do I look stoned?' " They used 420 as a code word for their activities and the time said activities would take place.

The group met in front of the statue of 19th-century French scientist Louis Pasteur, as well as other spots on school grounds, to get high at 4:20 p.m. It's said that the pack of teens would sometimes roam the campus, searching for a rumored marijuana patch.

The term "420" was widely in use by the end of the 1970s. Deadheads spread it outward like a virus from their San Rafael ground zero. Within a decade, pot smokers were using it across the country and around the world.