DENVER –  Students and others across the country have long observed April 20 as a day to celebrate marijuana. Here are some questions and answers about the counterculture holiday and the state of marijuana in the United States:
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WHO CELEBRATES? The observation is shared by marijuana users from San Francisco's Golden Gate Park to New York's Greenwich Village. Last year, some 10,000 people gathered at the University of Colorado-Boulder to simultaneously smoke marijuana. This year, the university is shutting down a campus quad that hosts the annual 4/20 gathering. Organizers say the protest may be moved to a nearby off-campus neighborhood, which could cause clashes between police and protesters. A rally is planned for Denver near the state capitol on Friday and Saturday. Police have suggested they'll be taking a hands-off approach to the gathering, which could draw tens of thousands of people. In Austin, Texas, country music legend Willie Nelson, who's open about his marijuana use, was expected to help unveil an 8-foot statue of himself in downtown Austin at 4:20 p.m. local time.

WHAT'S IT MEAN? The number 420 has been associated with marijuana use for decades, though its origins are murky. Its use as code for marijuana spread among California pot users in the 1960s and spread nationwide among followers of the Grateful Dead. Like most counterculture slang, theories abound on its origin. Some say it was once police code in Southern California to denote marijuana use (probably an urban legend). It was a title number for a 2003 California bill about medical marijuana, an irony fully intended. Others trace it to a group of California teenagers who would meet at 4:20 p.m. to search for weed (a theory as elusive as the outdoor cannabis crop they were seeking). Yet the code stuck for obvious reasons: Authorities and nosy parents didn't know what it meant.

HOW MANY STATES HAVE LEGALIZED MEDICAL MARIJUANA? In 1996, a ballot measure made California the first state to legalize medical marijuana. Sixteen states now allow the use of medicinal marijuana.

HOW MANY STATES ARE CONSIDERING LEGALIZING RECREATIONAL POT? Two states — Colorado and Washington — are considering legalizing pot for recreational purposes.

WHERE DOES THE FEDERAL GOVERNMENT STAND? Marijuana use, sale and possession are illegal under federal law.