It's April 20th, also known as 4/20, the day everyone associates with marijuana. So how did the number '420' become connected to smoking the illegal (in some states) substance?

It's NOT because 4-20 is a police code for a marijuana bust, like you might've heard. And it's not connected to Earth Day, which is two days later.

The consensus is...it's just a slang term from a group of stoners that managed to catch on.

Apparently, back in 1971, some kids at San Rafael High School in California used to meet outside the school to smoke weed every day at 4:20 P.M.

So their code to meet was '4-20.'  They eventually started using "4-20" to describe EVERYTHING related to marijuana and the term spread when they used it around traveling Grateful Dead fans, who adopted it.

Eventually, "High Times" magazine heard about the '4-20' term through the Dead fans, and took it to an even wider audience.  And since April 20th is 4/20, it became the unofficial holiday for weed smokers.

(Huffington Post)

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