orig jr brain on weed
Your brain on marijuana
01:39 - Source: CNN

Sign up for CNN’s Wonder Theory science newsletter. Explore the universe with news on fascinating discoveries, scientific advancements and more.

CNN  — 

And of course, on the calendar every April.

April 20, or 4/20, is known as “Weed Day” in some circles because the date corresponds with a numerical code for marijuana.

Jonathan Beaver of San Francisco holds a marijuana cigarette at the San Francisco Patients Cooperative, a medical cannabis cooperative, in San Francisco on November 29, 2004.

Yes, it seems arbitrary. So how did the number 420 come to represent smoking pot?

First, let’s get the myths and rumors out of the way:

The legend of the California penal code

Some claim the number is drawn from the California criminal codes used to punish the use or distribution of marijuana. But the state’s 420 code actually applies to obstructing entry on public land. So, not quite.

But the rumor sounds a lot like …

The legend of the police radio code

Neither the Los Angeles Police Department nor the New York Police Department even have a code 420. San Francisco Police have one, but it’s for a “juvenile disturbance.”

So never mind that theory.

Then there’s …

The legend of the Bob Dylan song

This one is a nod to Dylan’s song, “Rainy Day Women #12 & 35” and its lyric, “Everybody must get stoned.”

Multiply 12 by 35 and you get 420.

This theory seems a bit of a stretch. And Dylan himself has never confirmed any link.

The story that appears to hold the most water is …

The legend of the Waldos

What we know as 4/20 actually started as a secret code among high schoolers in the early 1970s, according to Chris Conrad, a court-qualified cannabis expert and instructor emeritus of political science at Oaksterdam University in Oakland, California.

A group of friends at San Rafael High School in Marin County, California, who called themselves “the Waldos,” would often meet at 4:20 p.m. to get high.

For them, it was an ideal time: They were out of school but their parents still weren’t home, giving them a window of unsupervised freedom. They met at that time every day near a statue of Louis Pasteur, the scientist who pioneered pasteurization.

The 4:20 time became a code for them to use in front of their unsuspecting parents, and 420 gradually spread from there — possibly via Grateful Dead followers — across California and beyond. The number is even used for a California Senate bill that established the state’s medial marijuana program.

What was shorthand for a group of friends can now be seen on T-shirts and throughout pop culture.

And of course, on the calendar every April.

This story was originally published in 2020 and has been updated.