420 Day is a term common in North America that refers to the use of marijuana and dates back to the early 1970s. Popular with teens and young adults, the 420 is usually a reference to 4:20pm and is commonly celebrated on April 20th.
420, 4:20, or 4/20 (pronounced four-twenty) is primarily a term used in North America and refers to the consumption of cannabis and, by extension, a way to identify oneself with cannabis subculture. Observances based on the number include the time (4:20 pm) as well as the date (April 20). The earliest use of the term began among a group of teenagers in San Rafael, California in 1971. Calling themselves the Waldos, because "their chosen hang-out spot was a wall outside the school," the group first used the term in connection to a fall 1971 plan to search for an abandoned cannabis crop that they had learned about. The Waldos designated the Louis Pasteur statue on the grounds of San Rafael High School as their meeting place, and 4:20 pm as their meeting time. The Waldos referred to this plan with the phrase "4:20 Louis".
Contributions by Kingturtle, Livajo, and Matt Gies.