The University of Colorado in Boulder was open to the public on 4/20, the unofficial pot holiday, for the first time in four years on Monday.

The school has blocked public access recently to snuff out one of the nation's largest marijuana celebrations. The crackdown eased this year, and campus remained open except for a gathering spot notorious for huge crowds of weed smokers.

Here are some things to know about how the holiday and its history in Colorado:

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NORLIN QUAD

About 10,000 people gathered to smoke pot on Norlin Quad across from the university's main library in 2011. The following year, officials closed the campus to everyone except students with identification cards. They also sprayed stinky fish-based fertilizer on the grass to discourage another big turnout. Undeterred, about 300 students gathered on another campus field, counted down the seconds to 4:20 p.m. and lit up. Hundreds of others watched the defiant smoke out. While the campus was open Monday, Norlin Quad was roped off and police warned they could ticket or arrest anyone who trespassed.

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BUSINESS AS NORMAL

In previous years, some teachers cancelled classes on April 20, anticipating students being absent in one way or another. With things back to normal, some students say they're too busy with school work to celebrate — at least on Monday. Junior Connor Dunn said he had two exams Tuesday so wouldn't be joining any smoke outs. His friend, Winston Heuga, also said he would be busy with homework. He spoke wistfully of the big celebrations he heard about before he arrived on campus, but he said there doesn't seem to be any interest in reviving them among his fellow students, most of whom arrived on campus after the crackdown.

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DENVER

Denver's observance of the holiday has spread into a multi-day festival that has become the new focal point of Colorado's celebration. Thousands of people gathered in Denver over the weekend for concerts, rallies and festivals. About 160 citations were issued, mostly for public consumption, which remains illegal under the state's recreational pot law, which was passed in 2012. Activists unhappy about the public ban on smoking plan to rally at 4:20 p.m. across from the state Capitol.

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4/20

The origins of the number 420 as a code for marijuana are murky. Fans of the drug have long marked April 20 as a day to enjoy pot — especially at 4:20 p.m. — and call for increased legal access to it. 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, a fully intended coincidence.