Commonly Accepted origin
Snopes.com, High Times magazine, The Marijuana-Logues, and The Straight Dope claim that in the early 1970s, a group of teenagers at San Rafael High School in San Rafael, California used to meet every day after school at 4:20 p.m. to smoke marijuana at the Louis Pasteur statue. One piece of evidence supporting an origin of the term from the time 4:20 is the fact that the number is always said "four twenty". This theory is also the most cited, and the most widely-accepted.
Other possible suggested origins
The Bob Dylan song "Rainy Day Women #12 & 35" contains the lyrics "everybody must get stoned" prominently throughout the song. Since twelve multiplied by thirty-five equals 420, it has been suggested that this song is the original source of the term 420 in cannabis culture.
Marijuana grew wild on or near Highway 420 in Ontario in the 1960s. However, Highway 420 only gained its current name in 1972. Whether or not this is true, Highway 420 is the location for an annual legalization protest every April 20th.
At some American junior or senior high schools, "after-school detention" ends at 4:20. Thus 4:20 signifies the time when the detainees are finally free to smoke after the school day.
The term could have possibly come from H. P. Lovecraft's "Within The Walls of Eryx" which contains the line, "My route must have been far from straight, for it seemed hours before I was free of the mirage-plant's pervasive influence... When I did get wholly clear I looked at my watch and was astonished to find that the time was only 4:20." This theory for the origin of 4:20 was first postulated on the official website for the rock band Tool.
Refuted urban legends
420 degrees Fahrenheit is the temperature at which marijuana burns, the primary method of use. (In reality, marijuana burns at or around 450 degrees.)
420 is a police code for a drug bust or for "marijuana smoking in progress," or that 4:20 is or was the shift change for the police. However, there is no 420 police code.
There are 420 chemicals in marijuana (there are actually 315).
April 21 is the last day on which one is supposed to plant cannabis seeds (with the variety of planting regions and cannabis genetics, no such date can logically apply to all growing operations).
April 20 is approximately the last frost in the American climate, making it time to move pot plants outdoors. In actual fact the date varies from year to year, and from region to region.
420 Day is celebrated as April 20th was the date of Bob Marley's death. Marley actually died on May 11, 1981.
An ounce of marijuana sold gram by gram at $15, equals $420. However, Marijuana is not normally sold at this price.
April 20th events and traditions
Many North American cities and colleges hold pro-marijuana rallies, or at least celebratory marijuana gatherings, on April 20. Many famous landmarks are also targeted for public marijuana use, such as Parliament Hill in Canada. The authorities' reaction to these gatherings varies from place to place, as shown by two prominent examples.
University of Colorado
The largest known celebration of this kind has been held annually for the past decade at the University of Colorado's Farrand Field in Boulder, Colorado, where protestors gather at 4:20 PM to smoke cannabis. In 2005, responding to negative press that year, University Police made their first attempt to disperse the gathering. They posted signs saying students were not allowed on the field that afternoon. After the field was overrun by an estimated 2000 or more students, police declined to issue any citations, as with past years. They did, however, turn the sprinklers on in an attempt to disperse the crowd. In another attempt to stop the celebration in 2006, the University put up signs telling students Farrand Field was closed for the afternoon, and even hired event staff to keep students from going onto the field. This, however, was a failed attempt, as thousands gathered around the field, only to storm it at around 4 o'Clock.
University of Vermont
It was a tradition for many years at the University of Vermont for a very large number of people to gather on the campus green and smoke openly in protest of marijuana prohibition laws. The protest was traditionally tolerated by the police, who viewed it as an act of civil disobedience and would merely observe to ensure the safety of participants and bystanders. The annual protest was squashed on April 20, 2002, however, due to a decree by incoming University President Daniel Fogel, who ordered campus police to maintain a strict, heavily armed presence on the campus green. Students were not allowed to stay on the green (for any reason) and students who attempted to do so were ID'd, searched and detained. Fogel has stated multiple times before and since that he was trying to clean up the "reputation" of the University. Since then, police wearing bullet-proof vests and armed with shotguns have been a staple of 4/20 celebrations on the green; and while the protest has gone forward on a much smaller scale, police now arrest any smokers and the tone has changed markedly from that of a peaceful, albeit defiant, protest to one of conflict and division between students, police, and administrators.