420 refers to the date April 20th, or 4/20 (American date format), and can be expanded to 20th of April @ 4:20pm.
This date (and time) is informally recognised globally as the international holiday for marijuana culture, where enthusiasts celebrate by smoking it (aka. Weed Day).
There are many conspiracies as to how and when this ritual came about, but the most plausible dates from 1971. This is when a group of five friends from San Rafael high school, in Northern California, came into possession of a hand-drawn map locating a nearby marijuana plot (that they incidentally never found). The called themselves ‘The Waldos’ because their meeting places was beside one of the school’s walls at 4:20pm. The group began using the coded term 420, (police code for ‘marijuana smoking in progress’), and soon others, including The Grateful Dead rock band (referenced on their flyers), began using this informal code. In 1990, some ‘Deadhead’ fans popularised it by handing out fliers encouraging people to partake in ‘420’ on April 20th @ 4:20pm. Thus the annual ‘stoner’ date was fixed and is celebrated worldwide by marijuana aficionados.
In recent times, Elon Musk, the world’s richest man, has made numerous coded references to 420. His first in a 2018 post declaring he was considering taking his EV-car company Tesla private at $420 per share. His latest in 2022, offering to purchase twitter at $54.20 per share.
One amusing anecdote is that the 420 mile marker on Interstate-70 in Colorado, had to be changed to show 419.9 miles, because all the 420 signs used to get pinched.