April 20 marks a high holiday for marijuana enthusiasts, who have long used "420" as code for cannabis and who celebrate the date's appearance on the calendar—on April 20, or 4/20—with concerts, stoner-film marathons and controlled substances.

This year, with the date falling on a Friday, marketers are trying to get in on the buzz. Marijuana advocates say the commercialization just proves that pot is gaining mainstream acceptance; there were some 17.4 million users in 2010, up from 14.4 million in 2007, according to government...