I dug into the history of the terminology and here is what I found.The earliest use of the term began among 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 pm 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 that the teens used to mean pot-smoking in general.

High Times editor Steven Hager wrote "Are You Stoner Smart or Stoner Stupid?" in which he called for 4:20 pm to be the socially accepted hour of the day to consume cannabis. He would go on to attribute the early spread of the phrase among Deadheads.

April 20 observances

April 20 has evolved into a counterculture holiday in North America, where people gather to celebrate and consume cannabis.  Some events have a political nature to them, advocating for the decriminalization of non-medical cannabis in the United States.

Observances have been held in San Francisco's Golden Gate Park near the Haight-Ashbury district, the University of Colorado's Boulder campus,  Ottawa, Ontario, at Parliament Hill and Major's Hil Montréal, Québec at Mount Royal monument  Vancouver, British Columbia at the Vancouver Art Gallery   Auckland, New Zealand at the Daktory and Dunedin, New Zealand, at University of Otago.

 

 

 

Video of last year's 4-20-2011 Celebration at CU/Boulder;