What ya smokin'

A day where people gather to celebrate and consumer cannabis, better known as marijuana. The term derived from a group of teenagers at a high school in San Rafael, California back in 1971, who would meet after school at 4:20 p.m. to smoke marijuana at the Louis Pasteur statue.

a word used to express the time or date in which you will meet up and smoke a whole fucking shitload of marijuana with all of your friends.
Also a holiday held on April 20 where everyone in the worlds celebrates the fact that they are raging potheads.

People on gaming platforms who think it's cool to put 420 in their name. It also applies to anyone with weed related names.