Both marijuana smokers and non-smokers recognize April 20 or 4/20 as a national holiday for cannabis culture, but few actually know how the date got chosen.
Some say “420” is code among police officers for “marijuana smoking in progress.” Some note 4/20 is also Adolf Hitler’s birthday. And some go as far as to cite Bob Dylan’s song “Rainy Day Women #12 & 35” because 12 multiplied by 35 equals 420. But, to put it bluntly, those rumors of the history behind how April 20, and 4/20, got associated with marijuana are false.
The most credible story traces 4/20 to Marin County, Calif. In 1971, five students at San Rafael High School would meet at 4:20 p.m. by the campus’ statue of chemist Louis Pasteur to partake. They chose that specific time because extracurricular activities had usually ended by then. This group — Steve Capper, Dave Reddix, Jeffrey Noel, Larry Schwartz, and Mark Gravich — became known as the “Waldos” because they met at a wall. They would say “420” to each other as code for marijuana.
read more here: Time.com