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.