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BOULDER, CO - APRIL 20:  The haze of marijuana smoke looms over a crowd of thousands  at 4:20 pm April 20, 2010 at the University of Colorado in Boulder, Colorado.   April 20th has become a de facto holiday for marijuana advocates, with large gatherings and "smoke outs" in many parts of the United States.  Colorado, one of 14 states to allow use of medical marijuana, has experienced an explosion in marijuana dispensaries, trade shows and related businesses in the last year as marijuana use becomes more mainstream here.  (Photo by Chris Hondros/Getty Images) Chris Hondros