Eh..I got this from wikipedia.org

Quote Originally Posted by www.wikipedia.org
420 (pronounced four-twenty) is a term used in North America as a discreet way to refer to cannabis and, by extension, a way to identify oneself with Cannabis culture. It is widely accepted that in the early 1970s, a group of teenagers at San Rafael High School in San Rafael, California used to meet every day after school at 4:20 p.m. to smoke cannabis at the Louis Pasteur statue.

Cannabis users gather on April 20 ("4/20" in the United States dating shorthand) to celebrate and consume cannabis. "420 friendly" is sometimes seen on advertisements for roommates, indicating that the current occupants are tolerant of cannabis users.
Andrew Nguyen Reviewed by Andrew Nguyen on . Origin of 420? Why 420? Where did it come from? Why not like... 513? Rating: 5