Did you Google 420?
Per wiki-
"
420,
4:20 or
4/20 (pronounced four-twenty) is cannabis culture slang for marijuana and hashish consumption, especially smoking around the time
4:20 pm (16:20). It also refers to cannabis-oriented celebrations that take place annually on April 20 (
4/20 in U.S. form).
At locations in the United States where cannabis is legal,
cannabis dispensaries will often offer discounts on their products on April 20.
[5]"
Check out the article, there's a lot more info
https://en.wikipedia.org/wiki/420_(cannabis_culture)