420, 4:20 or 4/20 (pronounced four-twenty) refers to consumption of cannabis and, by extension, a way to identify oneself with cannabis subculture. The notable day for these is April 20.
April 20 (4/20 in U.S. date notation) has evolved into a counterculture holiday, where people gather to celebrate and consume cannabis. Some events have a political nature to them, advocating for the decriminalization of non-medical cannabis in the United States.
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There is a persistent myth that that all the clocks in the movie Pulp Fiction are set to 4:20 (although, certainly all the clocks on the wall in the pawn shop are set to 4:20). However, in at least two scenes it is obvious that this is not the case. In the "Bonnie Situation" while Jimmy, Vince and Jules are drinking coffee in the kitchen, the clock clearly reads 8:15. Secondly, when Vince and Jules go to retrieve the briefcase, it is "7:22 in the a.m.". The significance of the time 4:20 is that it is slang for smoking marijuana.