Cannabis Culture

The term was allegedly coined by a group of teenagers in San Rafael, California in 1971. Calling themselves the Waldos, because "their chosen hang-out spot was a wall outside the school," the group first used the term in connection to a fall 1971 plan to search for an abandoned cannabis crop that they had learned about. The Waldos designated the Louis Pasteur statue on the grounds of San Rafael High School as their meeting place, and 4:20 p.m. as their meeting time. The Waldos referred to this plan with the phrase "4:20 Louis". Multiple failed attempts to find the crop eventually shortened their phrase to simply "4:20", which ultimately evolved into a codeword the teens used to mean pot-smoking in general.

High Times Creative Director Steven Hager was the first person to track down the Waldos and publish their account of the origins of the term. Hager wrote "Are You Stoner Smart or Stoner Stupid?" (October 1998) in which he called for 4:20 p.m. to be the socially accepted hour of the day to consume cannabis. "I believe 420 is a ritualization of cannabis use that holds deep meaning for our subculture," wrote Hager. "It also points us in a direction for the responsible use of cannabis."

Some also insist that the original meaning comes from Bob Dylan's song "Rainy Day Woman #12&35". Released in 1966, it pre-dates the 'Waldos' story by years. The tell-tale lyrics "But I would not feel so all alone--everybody must get stoned!" and the fact that 12 multiplied by 35 equals 420 points to the veracity of this origin.

http://en.wikipedia.org/wiki/420_%28cannabis_culture%29