The day to smoke weed. It is based off of what police say when they see people doing drugs ("We have a 4/20 on...").
It is celebrated internationally and people also wear green on that day. It is also Adolf Hitler's birthday, and the date f the Columbine School shootings. Pronounced "four twenty."
Yo man, I got some fresh Columbian weed for us to smoke tomorrow... after all, it is 4/20.