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.