For marijuana users, 4/20 is code for the day to openly celebrate the illegal drug.
Searches on "420" have been lighting up on Yahoo! and in the twitterverse, with "4/20" and "Mary Jane Day" becoming trending topics.
In Austin, Texas, Willie Nelson (a famous fan of the herb) will help unveil a statue of himself at, you guessed it, 4:20 p.m.
At the University of Colorado at Boulder, the college is in a less celebratory mood. Officials will be putting a stop to the annual 4/20 rally that has involved 10,000 celebrants descending on the campus and lighting up.
The higher institution of learning has a reputation for being the top party school in the country, and no more so on 4/20, when people assemble to demand the legalization of the drug.
The university spokesman Bronson Hilliard told the Associated Press, "We don't consider this a protest. We consider this people smoking pot in the sunshine," adding, "This is a gathering of people engaging in an illegal activity."
Student leader Daniel Ellis Schwartz disagreed, saying the university's actions went against the students' First Amendment rights. "I do not see any justification for the university shutting it down." He added that closing the quad would merely push the event off-campus.
Since the 1960s, the pot-smoking holiday has been observed by the counter- culture everywhere from San Francisco's Golden Gate Park, to New York City's Greenwich Village.
The code for pot was spread by followers of the Grateful Dead, but the origins are murky. Urban legends abound: It was a police code in Southern California for marijuana use. It was the time a gang of teenagers would meet to search for weed. However it started, it stuck because square parents and authorities had no idea what it meant.
4/20 has become a pop-culture reference for those in the know: A clock in the movie "Pulp Fiction" is stuck at 4:20. Craigslist ads for roommates often ask for those who are "420 friendly." The publication "High Times" owns the domain 420.com.
As for the legalization battle, 16 states now allow the use of medical marijuana. But smoking and distributing pot is illegal under federal law. Washington state and Colorado are considering laws that would legalize recreational use.