17 April 2014 | BBC

The number 420 has become a popular code for marijuana. Where does the term come from and why did it catch on, asks Aidan Lewis.

On Sunday pot smokers will gather across the US to mark what has become a hallowed date in their calendar - 4/20, or 20 April - by smoking marijuana, possibly at 4:20pm.

The 4/20 celebrations have taken off in the last few years, but their origins appear to lie in the escapades of a group of friends from San Rafael high school, northern California, in 1971. That autumn, the five teenagers came into possession of a hand-drawn map supposedly locating a marijuana crop at Point Reyes, north-west of San Francisco.

The friends - who called themselves the Waldos because they used to hang out by a wall - met after school, at 4:20pm, and drove off on their treasure hunt. They never found the plot. "We were smoking a lot of weed at the time," says Dave Reddix or Waldo Dave, now a 59-year-old filmmaker. "Half the fun was just going looking for it." The group began using the term 420. So did friends and acquaintances, who included - at a couple of steps removed - members of the Grateful Dead rock band. The term spread among the band's fans, known as Deadheads.

Then in 1990 Steve Bloom, an editor at High Times, saw 420 explained on a Grateful Dead concert flyer. Staff on the magazine, long the leading publication on marijuana, started using it. (They held ideas meetings at 4.20pm - pot-fuelled, of course.) Twenty years later another publication, 420 Magazine, reported a claim by a rival group of San Rafael old boys that they had invented the term. But the Waldos, who have shown letters and other items to High Times, vigorously defend their version. "We're the only ones with evidence," says Steve Capper, or Waldo Steve.

Bloom says the term has served as a sort of semi-private code, and cannabis smokers tend to spot it everywhere - building numbers, prices, even clocks in the film Pulp Fiction. After the 420-mile marker on the Interstate-70 highway in Colorado was repeatedly pinched, officials recently replaced it with a 419.99-mile sign.

This year Denver will be the epicentre of festivities, thanks to Colorado recently becoming the first state to permit the sale of recreational marijuana. Smokers are celebrating breakthroughs in their legalisation campaign elsewhere too. "This might be the biggest 420 ever," says Bloom. "This might be the peak of 420."

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