There have been a number of theories about the origin of cannabis' most celebrated day, known colloquially as 420 (spoken four-twenty). For years, one popular myth held that 420 was a police radio code for a cannabis related infraction. Another theory (long since debunked) suggested cannabis contained 420 chemicals, and still others have tried to link the cannabis celebration with the fact that it's also Hitler's birthday.
But the legend now generally accepted as the truth was first published in 2009 by Ryan Grim, Huffington Post's Washington Bureau Chief and author of the book This Is Your Country On Drugs. Grim's piece has been published each year on April 20.
The story goes that a group of high school students in Marin County, California, were planning to meet up and search for a plot of wild cannabis, which had been supposedly abandoned by a Coast Guard service member. It was 1971, a time before cellphones and the internet, and word was passed among the students that they would meet to begin the search at 4:20 pm, near a statue of Louis Pasteur.
The search for the plot of cannabis began at 4:20
The first search was unsuccessful, but subsequent ones were arranged, with the students meeting at 4:20 pm each time. Eventually, just speaking "four-twenty" among the group of students became code for meeting up for any activity related to 420.
The students were known as The Waldos, owing to their penchant for hanging out by a wall. In 2012, Grim revealed the names of the members and provided compelling proof that they were, in fact, responsible for creating 420. One member of The Waldos name was Mark Gravitch, whose father handled real estate transactions for The Grateful Dead.
The Waldos were hanging out at Grateful Dead practices, and the band, known for incessant touring, took up the cause and terminology of 420, and helped popularize it. This provided the seeds of what would become a global celebration of cannabis culture centered around the term 420 and the date April 20th.
h/t Huffington Post