Thursday marks marijuana culture’s high holiday, 4/20, when cannabis fans gather in clouds of smoke at music festivals, celebrate with all-you-can-deals on chicken wings and other munchies, and take advantage of pot-shop discounts in legal weed states.

This year’s edition provides an occasion for activists to reflect on how far their movement has come, with recreational pot now allowed in 21 states and the nation’s capital, as well as a national political climate that hasn’t moved as quickly on legalization as many expected.

Here’s a look at the holiday’s history.

Why 4/20

The origins of the date, and the term “420” generally, were long murky. Some claimed it referred to a police code for marijuana possession or that it arose from Bob Dylan’s “Rainy Day Women No. 12 & 35,” with its refrain of “Everybody must get stoned” — 420 being the product of 12 times 35.

But a consensus has emerged that it started with a group of bell-bottomed buddies from San Rafael High School in California, who called themselves “the Waldos.” A friend’s brother was afraid of getting busted for a patch of cannabis he was growing in the woods at Point Reyes, so he drew a map and gave the teens permission to harvest the crop, the story goes.