in the spirit of this day lets copy and paste from a Phish messageboard via craigslist regarding the meaning of this special day.
420 is a phreak’s (and not just a hippie’s) favorite number for a variety of reasons, or maybe for no reason at all, but colloquially the number says pot — “let’s smoke pot”, or “someone’s smoking pot”, or “gee, i really like pot”, or “time to smoke pot”, either by time (4:20 a.m. or p.m.), date (April 20th), or otherwise (e.g. State Route 420).
April 20th at 4:20 is marked by annual events in Mount Tamalpais, CA (an informal gathering); Marin Conty, CA (the 420 Hemp Fest); Ann Arbor, MI (the Hash Bash); and Washington, D.C. (buildup towards the July 4th Smoke-In).
Conventional wisdom: The most common tale is that 420 is the police radio code or criminal code (and therefore the police “call”) in certain part(s) of California (e.g. in Los Angeles or San Francisco) for having spotted someone consuming cannabis publicly, i.e. “pot smoking in progress”; that local cannabis users picked up on the code and began celebrating the number temporally (esp. 4:20 a.m., 4:20 p.m., and April 20); that the number became nationally popularized in the late 1980s and, more ferverently, in the early- to mid-1990s; and is colloquially applied to a variety of relaxed and/or inspired contexts, including not only pot consumption but also a “good time” more generally (in contrast to the drug war surrounding).
Conventions are legends: 420 is not police radio code for anything, anywhere. Checks of criminal codes (including those of the City of San Francisco, the City of Los Angeles, Los Angeles County, the State of California, and the federal penal code) suggest that the origin is neither Californian nor federal (the two best guesses).
For instance, California Penal Code 420 defines as a misdemeanor the hindrance of use (“obstructing entry”) of public lands, and California Family Code 420 defines what constitutes a wedding ceremony (Marco).
One state does come close: “The Illinois Department of Revenue classifies the Alcoholic Liquor Act under Part 420, and the Cannabis and Controlled Substances Tax Act are next, under Part 428.” (RB 5/19/99)
True story?: “According to Steven Hager, editor of High Times, the term 420 originated at San Rafael High School, in 1971, among a group of about a dozen pot-smoking wiseacres who called themselves the Waldos. The term 420 was shorthand for the time of day the group would meet, at the campus statue of Louis Pasteur, to smoke pot.
“Waldo Steve,’ a member of the group who now owns a business in San Francisco, says the Waldos would salute each other in the school hallway and say “420 Louis!’ The term was one of many invented by the group, but it was the one that caught on. “It was just a joke, but it came to mean all kinds of things, like `Do you have any?’ or `Do I look stoned?’ ‘ he said. “Parents and teachers wouldn’t know what we were talking about.’ The term took root, and flourished, and spread beyond San Rafael with the assistance of the Grateful Dead and their dedicated cohort of pot-smoking fans. The Waldos decided to assert their claim to the history of the term after decades of watching it spread, mutate and be appropriated by commercial interests.
The Waldos contacted Hager, and presented him with evidence of 420’s history, primarily a collection of postmarked letters from the early ’70s with lots of mention of 420. They also started a Web site, waldo420.com.
“We have proof, we were the first,’ Waldo Steve said. “I mean, it’s not like we wrote a book or invented anything. We just came up with a phrase. But it’s kind of an honor that this emanated from San Rafael.'” – Maria Alicia Gaura for the San Francisco Chronicle, 4/20/00 p. A19
There are a variety of other explanations, all much more interesting than “police code”, and many plausible. Some are more likely uses of the 420/hemp connection rather than sources of it, such as the score for the football game in Fast Times at Ridgement High, 42-0.