"420" is perhaps the most well-known slang term in cannabis culture. It originated in the early 1970s with a group of high school students in San Rafael, California, who would meet after school at 4:20 p.m. to smoke weed. They used "420" as their secret code for marijuana. Over time, the term spread through Grateful Dead concerts and other counterculture events, becoming a universal code for weed. Now, April 20th (4/20) is celebrated as a sort of unofficial holiday for cannabis enthusiasts around the world, marked by smoking sessions and public events.