Each year on 20 April marijuana advocates around the world, from Amsterdam to California, celebrate cannabis culture – whether it’s legal or not.

Last year more than 1,000 people gathered in London’s Hyde Park to mark 4/20, smoking joints together at the designated time while calling for cannabis to be legalised.

But why are the numbers 420 synonymous with cannabis culture?

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One urban myth suggested 420 was the penal code in the state of California used by police officers for marijuana use. But according to Snopes.com, the 420 penal code actually refers to “obstructing entry on public land,” and does not refer to marijuana use in any other US state, either.

The Huffington Post suggested the term 420 originated with a group of high school students in California in the 70s, and was later amplified through the subculture attached to Californian band the Grateful Dead and the publication High Times.

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The term was allegedly coined in 1971 when a group of five students from the San Rafael High School went on a hunt for a plot of cannabis plants that were supposedly growing near the Point Reyes Peninsula Coast Guard Station. They say they met at 4:20pm outside their school to drive to the area and carry out the search many times.

They never found the patch, but the numbers 420 did become a useful code for them to communicate with each other.

Where cannabis is and isn't legal Show all 10 1 /10 Where cannabis is and isn't legal Where cannabis is and isn't legal UK Having been reclassified in 2009 from a Class C to a Class B drug, cannabis is now the most used illegal drug within the United Kingdom. The UK is also, however, the only country where Sativex – a prescribed drug that helps to combat muscle spasms in multiple sclerosis and contains some ingredients that are also found in cannabis - is licensed as a treatment Getty Where cannabis is and isn't legal North Korea Although many people believe the consumption of cannabis in North Korea to be legal, the official law regarding the drug has never been made entirely clear whilst under Kim Jong Un’s regime. However, it is said that the North Korean leader himself has openly said that he does not consider cannabis to be a drug and his regime doesn’t take any issue with the consumption or sale of the drug MARCEL VAN HOORN/AFP/Getty Images Where cannabis is and isn't legal Netherlands In the Netherlands smoking cannabis is legal, given that it is smoked within the designated ‘smoking areas’ and you don’t possess more than 5 grams for personal use. It is also legal to sell the substance, but only in specified coffee shops Getty Where cannabis is and isn't legal USA Although in some states of America cannabis has now been legalised, prior to the legalisation, police in the U.S. could make a marijuana-related arrest every 42 seconds, according to US News and World Report. The country also used to spend around $3.6 billion a year enforcing marijuana law, the American Civil Liberties Union notes AP Photo/Ted S. Warren Where cannabis is and isn't legal Spain Despite cannabis being officially illegal in Spain, the European hotspot has recently started to be branded, ‘the new Amsterdam’. This is because across Spain there are over 700 ‘Cannabis Clubs’ – these are considered legal venues to consume cannabis in because the consumption of the drug is in private, and not in public. These figures have risen dramatically in the last three years – in 2010 there were just 40 Cannabis Clubs in the whole of Spain. Recent figures also show that in Catalonia alone there are 165,000 registered members of cannabis clubs – this amounts to over 5 million euros (£4 million) in revenue each month Getty Where cannabis is and isn't legal Uruguay In December 2013, the House of Representatives and Senate passed a bill legalizing and regulating the production and sale of the drug. But the president has since postponed the legalization of cannabis until to 2015 and when it is made legal, it will be the authorities who will grow the cannabis that can be sold legally. Buyers must be 18 or older, residents of Uruguay, and must register with the authorities Getty Where cannabis is and isn't legal Pakistan Despite the fact that laws prohibiting the sale and misuse of cannabis exist and is considered a habit only entertained by lower-income groups, it is very rarely enforced. The occasional use of cannabis in community gatherings is broadly tolerated as a centuries old custom. The open use of cannabis by Sufis and Hindus as a means to induce euphoria has never been challenged by the state. Further, large tracts of cannabis grow unchecked in the wild Getty Where cannabis is and isn't legal Portugal In 2001, Portugal became the first country in the world to decriminalize the use of all drugs, and started treating drug users as sick people, instead of criminals. However, you can still be arrested or assigned mandatory rehab if you are caught several times in possession of drugs Getty Where cannabis is and isn't legal Puerto Rico Although the use of cannabis is currently illegal, it is said that Puerto Rico are in the process of decriminalising it RAUL ARBOLEDA/AFP/Getty Images Where cannabis is and isn't legal China Cannabis is grown in the wild and has been used to treat conditions such as gout and malaria. But, officially the substance is illegal to consume, possess and sell Getty

“I could say to one of my friends, I’d go, ‘420,’ and it was telepathic. He would know if I was saying, ‘Hey, do you wanna go smoke some?’ Or, ‘Do you have any?’ Or, ‘Are you stoned right now?’ It was kind of telepathic just from the way you said it,” Steve Capper, one of the five, told The Huffington Post.

“Our teachers didn’t know what we were talking about. Our parents didn’t know what we were talking about.”

The five friends – nicknamed the Waldos – hung out with the band the Grateful Dead, and the use of 420 as a code for cannabis began to spread through the band’s fans and its attached subculture. Journalist Steve Bloom writes on Celebstoner.com that in 1990, he attended one of band’s shows in Oakland, California, and was handed a flyer that allegedly told the story of 420.

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The flyer stated that 420 “started somewhere in San Rafael, CA in the late 70s,” but included the incorrect information that “it started as the police code for Marijuana Smoking in Progress”. It did, however, include a call for people to gather together at 4:20pm on 4/20 – the American style for writing 20 April, and smoke a joint as part of a “day of celebration”.

Mr Bloom writes that 420 was “a bit of stoner slang I’d never heard of,” and published the flyer in the May 1991 issue of High Times.

Steve Hager, then editor of the publication, told the Huffington Post that he “started incorporating it into everything we were doing,” and built a number of events around 420, from the World Hemp Expo Extravaganza to the Cannabis Cup.