420 Orgin

4/20 isn't the date Bob Marley died
420 isn't the number of a bill in U.S. Congress aiming to legalize weed
420 isn't the police or penal code for marijuana possession in California
Nor is it is the number of chemical compounds in cannabis, the pot plant, though lots of folks are fond of stating so.

420, 4:20, or 4/20 (pronounced four-twenty) is a code-term in cannabis culture that refers to the consumption of cannabis, especially smoking cannabis around the time 4:20 p.m./a.m. (or 16:20 in 24-hour notation) and smoking and celebrating cannabis on the date April 20 (which is 4/20 in U.S. form)

https://giphy.com/search/420

In 1971, Steve Capper, Dave Reddix, Jeffrey Noel, Larry Schwartz, and Mark Gravich, five high school students in San Rafael, California calling themselves the Waldos because "their chosen hang-out spot was a wall outside the school", used the term in connection with a fall 1971 plan to search for an abandoned cannabis crop that they had learned about, based on a treasure map made by the grower. The Waldos designated the Louis Pasteur statue on the grounds of San Rafael High School as their meeting place, and 4:20 p.m. as their meeting time. The Waldos referred to this plan with the phrase "4:20 Louis". Several failed attempts to find the crop eventually shortened their phrase to simply "4:20", which ultimately evolved into a codeword that the teens used to mean marijuana-smoking in general.

The catchphrase spread beyond their immediate circle, beyond the high school they attended and ultimately beyond California, so that within a decade or two pot smokers were using it across the country, and indeed the world over.

https://giphy.com/search/420

Mike Edison says that Steven Hager of High Times was responsible for taking the story about the Waldos to "mind-boggling, cult-like extremes" and "suppressing" all other stories about the origin of the term. Hager wrote "Stoner Smart or Stoner Stupid?" in which he called for 4:20 p.m. to be the socially accepted hour of the day to consume cannabis. He attributes the early spread of the phrase to Grateful Dead followers, who were also linked to the city of San Rafael.

Waldos, and the 420 flag. Created in San Rafael, California in 1971 Photo is from this 2012 article

Waldo's official website - https://420waldos.com/

source: https://en.wikipedia.org/wiki/420_(cannabis_culture)
http://manu133.deviantart.com/art/The-Waldos-484602740