The short answer is no one knows. But there are theories:
The legend of the Waldos
According to Chris Conrad, curator of the
Oaksterdam Cannabis Museum in Oakland, California, 420 started as a secret code among high schoolers in the early 1970s.
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. It's even the number of 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, in Tinder bios (420 friendly) and throughout pop culture.
And of course, on the calendar every April.