Over the past two decades, 21 states plus the District of Columbia and Guam have legalized marijuana for recreational use. That means marijuana smokers can smoke more openly than ever before on 4/20—a date that both marijuana smokers and non-smokers recognize as a national holiday for cannabis culture. Yet 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.