Tens of thousands of Americans will gather in parks and city plazas under a fog of marijuana smoke on Wednesday to celebrate a drug that remains illegal in much of the country.
April 20th has become synonymous with pot, a day when THC-infused clouds drift, people play bongos, and everyone feels a bit of love for each other.
But why did 4/20 become so important?
What is 4/20?
Basically a day that celebrates all things Mary Jane.
In more than a dozen US states where weed is legal, big rallies will take place in public spaces, with participants lighting up everything from regular spliffs to bongs to joints the size of a baby's arm. If you can imagine it, there's someone smoking it.
Outside of the big gatherings, smaller groups of people will get together to indulge in a collective toke.
Aficionados insist these more intimate smoke-ins are more in keeping with the origins of 4/20, which began as something off-the-grid but has grown into full-size festivals, complete with sponsors and the option of purchasing VIP tickets.
Why April 20th?
In the US system of dates, April 20 is 4/20, and the day-long celebration of reefer seems to have metamorphosed from the number.
There are a few competing theories where the number came from, and none of them is commonly accepted.
One holds that 420 is the number of chemicals in marijuana, though this seems to be an undershoot (it's closer to 500, according to Vox), or an overshoot ("scientists have successfully isolated 113 discrete cannabinoids," weed bible High Times reported.)
Another theory has it that 420 is the California penal code for possession of the drug (it's not; Penal Code 420 explains that it's a misdemeanor to prevent access to public lands in the state, according to findlaw.com, a website for legal professionals).
Many suggestions involve well-known pot heads The Grateful Dead in some way, and include the claim that 420 was the favored hotel room number on their psychedelic tours. Was it implausible for all six musicians (plus hangers-on) to be in the same room? Yes, and a band spokesman is widely quoted as saying it's not true.
Nor is 4/20 the date that Jim Morrison, Jimi Hendrix, or Janis Joplin died. While they all passed at age 27, their deaths were in July, September and October, respectively.
One of the more satisfying theories about 4/20 is that 4:20pm was the time that a group of stoners in 1970s California would gather to get high. The Waldos describe themselves as "a group of five wisecracking friends...who originated the term '420' in 1971 at San Rafael High School."
Their website offers what they say is proof of the term being used as code for these gatherings and the act of smoking, in the form of letters exchanged after they left school.
Whatever the truth, the term has entered the lexicon -- the Oxford English Dictionary acknowledges that it is "North American slang" for marijuana, while the Encyclopaedia Britannica, no less, notes the expression is "in current use" (and also gives us "hippie lettuce" as an alternative term for the drug).