Officials at the University of Colorado-Boulder are trying to prevent thousands of people from gathering on campus today for the annual 4/20 marijuana celebration. They have spread smelly fish-based fertilizer on campus as a deterrent. “We don’t consider this a protest. We consider this people smoking pot in the sunshine,” university spokesman Bronson Hilliard told the Associated Press. “This is a gathering of people engaging in an illegal activity.”

April 20th is the unofficial marijuana day, when some people around the world celebrate cannabis and rally to push for laws to legalize marijuana. Those marking the occasion traditionally light up at 4:20 p.m. The reason the number 420 is linked with marijuana is unclear, the article notes.

Last year, more than 10,000 people, both students and non-students, attended a 4/20 celebration on the University of Colorado-Boulder campus, the AP reports. Today, officials are closing the campus to all unauthorized visitors, and are offering a free campus concert by hip-hop star Wyclef Jean to coincide with the appointed hour.

Colorado was named the nation’s top party school in 2011 by Playboy magazine, and ranks among the top schools for marijuana use.

Marijuana is a hot topic in Colorado. Medical marijuana is legal in the state. Recently, the owners of 25 medical marijuana dispensaries located within 1,000 feet of a school received letters from Colorado U.S. Attorney John Walsh telling them that they must close, or potentially face criminal prosecution.

The state will vote on a ballot initiative in November that potentially could legalize recreational marijuana use. Medical marijuana is legal in 16 states and the District of Columbia, but the drug remains illegal under federal law.