4:20

The origin of the term stems from a story about a group of teenagers at San Rafael High School in San Rafael, California, United States in 1971.[1][2] The teens would meet after school at 4:20 p.m. to smoke marijuana at the Louis Pasteur statue. The exact time was chosen because that was the time that afternoon detention was dismissed.

http://en.wikipedia.org/wiki/420_(cannabis_culture)