Police blocked off Norlin Quad at the University of Colorado in Boulder, in April 2012 to prevent the annual 4/20 celebration. 

(AP Photo/Ed Andrieski)

The University of Colorado Boulder campus will be open to visitors Monday, a change from the past three years when officials shut down the campus to try to end the annual 4/20 marijuana celebration. 

In 2011, the annual pot event attracted about 10,000 people. University of Colorado spokesman Ryan Huff said the event had grown out of control.

"We had several complaints from people who didn’t like the marijuana smoke being there," Huff said. "They wouldn’t even come to work that day, so it really caused hardships to our academic mission."

That led to the visitor ban. But now officials believe they’ve ended the 4/20 tradition, so the campus will be open for the day.

"We thought it was time to allow visitors back on campus and have as normal of a school day as we can," he said. 

The traditional 4/20 gathering spot will not be open, however. Police officers will cite anyone on the Norlin Quadrangle with trespassing.

In Denver, activists plan to protest the official ban on smoking marijuana in public with a rally at 4:20 p.m. near the Capitol. Thousands of people gathered there for pot celebrations over the weekend, with police issuing about 160 citations.

The Associated Press contributed to this report.