420 was a police code, I believe in a California city, way back in the day. 420 was the code they used for person/people who were smoking marijuana. The term has been used by most every American teenager. Many people use it as a time to smoke. I know of people who consider 4/20 (April 20) a holiday of sorts.
"It's 4:18 roll the blunt(joint)/pack the bowl(bong,pipe) so we're ready for 420"