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 water tower. 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" and not "four hundred twenty." This theory is also the most cited, and the most widely-accepted.
Urban legends
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.
There are 420 chemicals in marijuana.
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, making it time to move pot plants outdoors.
At 4:20 on an analog clock, the hands line up in a downward angle giving the appearance that the face of the clock has a smoking instrument in its "mouth."
Marijuana grew wild on or near a purported Highway 420 in the western United States in the 1960s.
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.
420 degrees Fahrenheit is the temperature at which marijuana burns, the primary method of use.
The term came 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".