One explanation of the origin of the term stems from a story about a group of teenagers at San Rafael High School in San Rafael, California, United States in 1971.[1] The teens would meet after school at 4:20 p.m. to smoke marijuana at the Louis Pasteur statue. The exact time was chosen because that was the time that afternoon detention was dismissed.

1. "Marijuana Advocates Point to Signs of Change". New York Times. April 19, 2009. Retrieved on 2009-04-20. "Mr. Hager said the significance of April 20 dates to a ritual begun in the early 1970s in which a group of Northern California teenagers smoked marijuana every day at 4:20 p.m. Word of the ritual spread and expanded to a yearly event in various places. Soon, marijuana aficionados were using ?420? as a code for smoking and using it as a sign-off on fliers for concerts where the drug would be plentiful. ... In recent years, the April 20 events have become so widespread that several colleges have urged students to just say no. At the University of Colorado, Boulder, where thousands of students regularly use the day to light up in the quad, administrators sent an e-mail message this month pleading with students not to ?participate in unlawful activity that debases the reputation of your university and degree.?"