More than 50,000 are expected to light up outdoors in Denver's Civic Center Park on April 20 to celebrate marijuana legalization. Thousands more are headed here for the nation's first open-to-all Cannabis Cup, April 20-21, a domestic version of an annual marijuana contest and celebration in Amsterdam. Expected guests at the Cannabis Cup, a ticketed event taking place inside the Denver Convention Center, include Snoop Lion, the new reggae- and marijuana-loving persona for the rapper better known as Snoop Dogg.

Marijuana activists from New York to San Francisco consider April 20 a day to celebrate the drug and push for broader legalization. The origins of the number "420" as a code for pot are murky, but the drug's users have for decades marked the date 4/20 as a day to use pot together.