420, 4:20, or 4/20 (pronounced four-twenty) is a term used primarily in North America to refer 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 p.m.) 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 p.m. as their meeting time. The Waldos referred to this plan with the phrase "4:20 Louis". Multiple failed attempts to find the crop eventually shortened their phrase to simply "4:20", which ultimately evolved into a codeword that the teens used to mean pot-smoking in general.
High Times editor Steven Hager wrote "Are You Stoner Smart or Stoner Stupid?" in which he called for 4:20 p.m. to be the socially accepted hour of the day to consume cannabis. He attributes the early spread of the phrase to Grateful Dead followers, who were also linked to the city of San Rafael. (Wikipedia)