Well...it seems to have started with the Greatful Dead.
It was Christmas week in Oakland, 1990. Steven Bloom was wandering through The Lot - that timeless gathering of hippies that springs up in the parking lot before every Grateful Dead concert - when a Deadhead handed him a yellow flyer.
"We are going to meet at 4:20 on 4/20 for 420-ing in Marin County at the Bolinas Ridge sunset spot on Mt. Tamalpais," reads the message, which Bloom dug up and forwarded to the Huffington Post. Bloom, then a reporter for High Times magazine and now the publisher of CelebStoner.com and co-author of Pot Culture, had never heard of "420-ing" before.The flyer came complete with a 420 back story: "420 started somewhere in San Rafael, California in the late '70s. It started as the police code for Marijuana Smoking in Progress. After local heads heard of the police call, they started using the expression 420 when referring to herb - Let's Go 420, dude!"
Here's three more articles at The Huffington Post. The first is from Norm Stamper, Retired Seattle police chief, member of Law Enforcement Against Prohibition, the next one is from Ryan Grimm, a writer for The Huffington Post, and another from Anthony Papa the Communications specialist for the Drug Policy Alliance.
And now for a video from CelebStoner.com filmed at the University of Colorado last year.