The number 420 became associated with weed as a coded term that originated in the early 1970s with a group of high school students in San Rafael, California, who called themselves "the Waldos." They used "420" as a secret code to meet at 4:20 p.m. to smoke marijuana. The term eventually spread through the Grateful Dead fans and was popularized by publications like High Times magazine. April 20th (4/20 in U.S. date format) became an unofficial holiday for cannabis culture, celebrating marijuana consumption and advocacy for legalization. The term refers both to the time (4:20 p.m.) and the date (April 20) associated with cannabis use.
