* 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". The song 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.
* Adolf Hitler was born on April 20. One smoker explained: "Hitler represents in sharp opposite contrast all that the marijuana-smoking community stands for." 
* In some school's code of conduct, rule 420 states illegal substances are not allowed on school grounds.
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 283 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.