420, 4:20, or 4/20 (pronounced four-twenty) is cannabis culture slang for marijuana consumption. Especially smoking around the time 4:20 pm.  Also refers to cannabis-oriented celebrations that take place annually on April 20 (which is 4/20 in U.S. form)

In 1971, five high school students in San Rafael, California, used the term “4:20” in connection with a plan to search for an abandoned cannabis crop. Based on a treasure map made by the grower. Calling themselves the Waldos,^[9][10] because their typical hang-out spot “was a wall outside the school”.^[11] The five students (Steve Capper, Dave Reddix, Jeffrey Noel, Larry Schwartz, and Mark Gravich)^[12] designated the Louis Pasteur statue^[13] on the grounds of San Rafael High School as their meeting place. And 4:20 pm as their meeting time.^[11] The Waldos referred to this plan with the phrase “4:20 Louis”. After several failed attempts to find the crop, the group eventually shortened their phrase to simply “4:20”. Which ultimately evolved into a code-word the teens used to refer to consuming cannabis.^[7]

Steven Hager of High Times was responsible for popularizing the story of the Waldos.^[14] The first High Times mention of 4:20 smoking and a 4/20 holiday appeared in May 1991,^[15]and the connection to the Waldos appeared in December 1998. Hager attributed the early spread of the phrase to Grateful Dead followers^[16] – after “Waldo” Reddix became a roadie for the Dead‘s bassist, Phil Lesh^[12] – and called for 4:20 pm to be the socially accepted hour of the day to consume cannabis. 420 too. Also. In conclusion.