Why is the number 420 used for pot?
One common belief is that 420 was the California police or penal code for marijuana, but there's no evidence to support those claims.
Another theory is that there are 420 active chemicals in marijuana, hence an obvious connection between the drug and the number. But there are more than 500 active ingredients in marijuana, and only about 70 or so are cannabinoids unique to the plant, according to the Dutch Association for Legal Cannabis and Its Constituents as Medicine.
A less-known explanation comes from the 1939 short story "In the Walls of Eryx" by HP Lovecraft and Kenneth Sterling. The story describes "curious mirage-plants" that seemed fairly similar to marijuana and appeared to get the narrator high at, according to his watch, around 4:20. Since the story is from 1939, it's perhaps the earliest written link between marijuana and 420.
Steven Hager, a former editor of the marijuana-focused news outlet High Times, told the New York Times that the holiday came out of a ritual started by a group of high school students in the 1970s. As Hager explained, a group of Californian teenagers ritualistically smoked marijuana every day at 4:20 pm. The ritual spread, and soon 420 became code for smoking marijuana. Eventually 420 was converted into 4/20 for calendar purposes, and the day of celebration was born. Still, there's little evidence to prove Hager's story beyond the claims of a group of Californians who took credit for 420's origin.