To the uninitiated, 420 (or 4:20 or 4/20) is a number with a strong association with cannabis (see the bottom of this page for why), hence April 20, or 4/20 in American date-keeping, has become a day for smoking and celebrating weed.
But it's not just the date which brings the stoners out around the world – including Hyde Park in London – to show off their pride in getting high. The number itself, in any form, is an attraction for pot smokers.
Indeed one hotel at least has taken to indicating its room 420 with a sign that reads 419+1 in a bid to avoid links with marijuana.
Photos taken by guests also show the room signs have no-smoking symbols on them, though this might be true of all the rooms in the hotel.
The motivation for creating the 419+1 sign might be two-fold: first, to discourage people wanting to smoke the drug in the famed room 420, and second to discourage people from stealing the label as memorabilia.
Another hotel in Colorado, where it is legal to smoke weed, decided to stencil 420 onto the corresponding door to save the trouble of replacing a placard each time it is stolen.
While another establishment just missed out the number altogether, skipping from 419 to 421.
Street signs bearing the number are another popular source of memorabilia, leading transport authorities in the likes of Colorado and Idaho to abandon 420 road markers in favour of 419.99 and, in the case of Minnesota, 42x.
But 420 is not only the number compelling hotels to act.
Room 13, due its association with the long-foreboding number, is often skipped by hotels, while buildings will often leave out the 13th floor, moving straight to 14 or designating the 13th floor as an out-of-bounds maintenance or technical floor.
The room number associated with the haunted abode in Stephen King's The Shining is 217, though film director Stanley Kubrick changed it to 237 for the film adaptation of the book.
The change, according to Wired.com, was because Mr Kubrick did not want to scare people from staying in room 217 at the Timberline Lodge in Oregon, which was used for hotel exterior shots in the film. Though, apparently, if you call the hotel, there is no such room.
There is, however, a room 217 at the Stanley Hotel, which inspired Mr King to write the book after he stayed there with his wife. The room, and hotel, has become an attraction for fans of the horror story.
EXPLAINER: Why 4/20?
Why do cannabis users celebrate marijuana on April 20, and at 4.20pm? There are numerous theories behind the origins of 4/20, including:
- It's the day Bob Marley's died. This is not true; he died on November 5. April 20 is not his birthday, either, but it is Adolf Hitler's birthday (historians remain divided about the extent of the Nazi leader's weed consumption).
- It's the police radio code for "weed smoking in progress". False. Police insist 420 means absolutely nothing.
- It refers to the number of chemical compounds in marijuana. Not true - there are reportedly 315.
- It's all about Bob Dylan. In his song "Rainy Day Women ♯12 & 35" he insisted that "Everybody must get stoned". 12 x 35 = 420.
The real reason, or at least the one that most people seem to agree on, is as follows. In the Seventies, a group of high school students in San Rafael, California, would routinely meet up for a sneaky joint by a statue of Louis Pasteur on campus at exactly 4.20pm. "Louis 420", they would whisper to one another in the halls. The code word spread inexorably and was picked up by weed bible High Times, which now owns the website 420.com, in the Nineties.
The Telegraph , London