“420” day is being celebrated in London today. What is 420?

Thousands of people assemble in London’s Hyde Park, as well as open places around the UK and beyond, to commemorate a day that has come to be associated with marijuana.

But how did this day become known as something of a sacred feast for marijuana users?

Although the origins of 420 are still questioned, the most widely acknowledged story comes from the 1970s in California.

“Back in 1971, five students at San Rafael High School in California would meet at 4.20pm – right after doing their after-school sports – by the school’s statue of Louis Pasteur to toke up,” according to a Hyde Park 420 Facebook event description.

The Waldos, as they were known, coined the term “420” to refer to the act of smoking marijuana.

“This spread, 4.20pm became known as the perfect time for smoking weed, and the number 420 became synonymous with marijuana,” the story goes.

“We would remind each other in the hallways we were supposed to meet up at 4.20.” In 2012 one of the group members, Steve Capper, told The Huffington Post.

Another member of the band, Dave Reddix, is said to have been acquainted with The Grateful Dead’s bassist. The band is credited with popularizing April 20th as “weed day.”

4/20 is April 20 in the American calendar, which puts the month before the day. As a result, April 20 became a day of celebration for all things marijuana-related.