Happy hour, High time, High noon

What does "420" mean?

Definition: A term used to denote periods during which the inhalation of marijuana vapors is promoted by the cannabis community. The term usually refers to the date, the 20th of April, 4/20/XXXX, or a time, 04:20 PM or AM.

Example usage for 420

"Load a bowl; it's 4:19!"