420 origins: It's not the police code for smoking marijuana
According to a 1998 article in High Times, the term "420" was coined by a group of teenagers in San Rafael, CA in 1971. They would meet up to search for an abandoned cannabis crop and their meeting code was 4:20 Louis, which means meet at 4:20 p.m. at the Louis Pasteur statue outside their school. It just snowballed from there.
An animated history of 420 that High Times put together is below and you can also check out their very helpful (and hilarious) Timeline of 420.
PICS: Celebs who smoke pot
While 420 will undoubtedly be quietly observed all over the country today, there are a few notable large gatherings every year. In San Francisco (natch), thousands of people will gather at Hippie Hill, near the famous Haight-Ashbury district. The University of Colorado Boulder and the University of California Santa Cruz each have large gatherings - in fact, the one in Boulder topped 11,000 people in 2008.
These events often become political in nature, as people are there also to advocate the legalization of marijuana in the United States.
Will you be celebrating 420?