Title text:
In 1899, people were walking around shouting ‘23’ at each other and laughing, and confused reporters were writing articles trying to figure out what it meant.
Transcript:
Transcript will show once it’s been added to explainxkcd.com
Source: https://xkcd.com/3184/
I’ve heard it said that 420 referred to the time 4:20 pm, when a group would come together to smoke, but that sounds contrived.
420 can also refer to the birth date of Adolf Hitler, which makes 420 a bit darker than just “haha, smoke.”