The Origins of 420

Most people know that 420 is a slang term for marijuana, or more accurately smoking marijuana. The question is where does the term come from. Many stories about the history of this term have circulated, in the end no one really knows for sure, however some are more likely than others, and some are downright ridiculous. I have my own beliefs as to where the term originated, but they are based on rumours that have been circulating for years, and a general application of Occam's Razor (the simplest solution is probably the correct one). So lets review the origin stories and find the best fit:

Police Code
One rumour holds that 420 is the police code for drug infraction by various police forces. To the best of my knowledge and searching I have never been able to find any clear evidence that 420 is used as any police code

Penal Code
It is said that marijuana use is covered by California Penal Code 420. Once again this is not the case. That being said there was a bill introduced into California Senate dealing with marijuana the bill was California Senate Bill 420 (HS 11362.7). However 420 was in common use by this time, so I think this can be discounted.

Number of Active Chemicals
Apparently there is a claim that there are 420 active chemicals in marijuana. This seems a little specious to me, it would be wonderful if this were true, however it is not. The number most often quoted for the number of active chemicals in marijuana is 315, although this does vary from strain to strain.

Birth/Death of Famous Pot User
There are several variants on this theory. Some purport that Jimi Hendrix, Janice Joplin, and/or Jim Morrison died, while other claim it is the birth of Hitler. So far as I know Hitler was not exactly a poster child for marijuana legalization, so I think we can discount that one, despite it being true that Hitler was born on April 20th. Jimi Hendrix died on September 18th, Joplin died on October 4th and Morrison died on July 3rd. I am sure other names have been thrown out there, but in the end I doubt this is the reason 420 became slang for pot use.

April 20th is the Best Time to Plant
Well, this is an odd one, best time to plant really relies on local, so it seems unlikely that this was the start. However it could be that in one region or another mid April is a good time to plant.

April 20th is National Pot Smoker's Day
True, but this occured because of the slang term 420, not the other way around.

Columbine
The Massacre at Columbine took place on April 20th. A regretable tragidy to be sure, however I fail to see how it could possibly be connected to the term 420.

Tea Time
One interesting theory holds that 4:20pm is tea time in Holland and thus the most common time for the Dutch to partake in a little toking with their tea (it is not oncommon for marijuana to be sold in tea/coffee shops in Amsterdam). This could certainly be an origin, however the saying was common in the United states, particularly western states first and spread outward from there. One would assume that the term would have been popular in Europe long before the United States if this was the case.

School's Out
Now for the theory I persoanlly believe. In the early 70's a group of San Rafael High School students used to meet behind a statue of Louis Pasteur after school let out, they would arive at 4:20pm for their smoke. it became common for them to discuss their pot smoking activity as partaking in 420. The timing fits, the location fits, and logically it makes sense. Slang almost always originates from youth and spreads out from there.