London is celebrating “420” day today. What does 420 stand for?

Thousands of people gather in London’s Hyde Park, as well as open spaces around the UK and beyond, to honour a day connected with marijuana.

But how did this day come to be considered as a sort of holy day among marijuana users?

Although the origins of 420 are still being debated, the most commonly accepted version dates back to the 1970s in California.

According to a Hyde Park 420 Facebook event description, “back in 1971, five students from San Rafael High School in California would assemble at 4.20pm — just after finishing their after-school activities – beside the school’s monument of Louis Pasteur to toke up.”

The Waldos, as they were affectionately called, created the phrase “420” to describe the act of smoking marijuana.

“As word spread, 4.20pm became recognised as the ideal hour for smoking dope, and the number 420 became synonymous with marijuana,” according to the tale.

“In the halls, we’d remind each other that we were scheduled to meet at 4.20.” Steve Capper, one of the group’s members, told The Huffington Post in 2012.

Dave Reddix, another member of the band, is reported to have known The Grateful Dead’s bassist. The band is credited with popularising the 20th of April as “weed day.”

In the American calendar, 4/20 is April 20, putting the month before the day. As a result, April 20 has become a day to celebrate anything marijuana-related.