Both marijuana smokers and non-smokers recognize April 20 or 4/20 as a national holiday for cannabis culture. This year’s celebration might be even more enthusiastic considering support for marijuana legalization is at an all-time high. But few actually know how the date got chosen.

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 a campus’ statue to partake. They chose that specific time because extracurricular activities had usually ended by then, but it wasn’t time to go home. This group became known as the “Waldos” because they would always meet at a wall on campus. They would say “420” to each other as code for marijuana.

As one of the originals told TIME in 2017, “We got tired of the Friday-night football scene with all of the jocks. We were the guys sitting under the stands smoking a doobie, wondering what we were doing there.”

Later, one of the guy’s brother helped him get work with Grateful Dead bassist Phil Lesh as a roadie. The band is said to have helped popularize the term “420” when on Dec. 28, 1990, a group of Deadheads in Oakland handed out flyers that invited people to smoke “420” on April 20 at 4:20 p.m. One ended up with Steve Bloom, a former reporter for High Times magazine, an authority on cannabis culture. The magazine printed the flyer in 1991 and continued to reference the number. Soon, it became known worldwide as code for marijuana. In 1998, the outlet acknowledged that the “Waldos” were the inventors of 420.

Bloom, now the publisher of Celebstoner.com, has credited the people who wrote the flyer for the date’s reputation as an annual gathering of pot smokers. “They wanted people all over the world to get together on one day each year and collectively smoke pot at the same time,” he wrote in 2015. “They birthed the idea of a stoner holiday; which April 20 has become.”

Happy 420 every day!

By Karen Watts Nauman