April 20, better known to some as "420," is unofficially known as a day for smoking and advocating the legalization of marijuana.

At the University of Colorado Boulder, one of the biggest pro-pot rallies is held annually at 4:20 p.m. on the campus' Norlin quad, reported CNN. Last year, more than 10,000 people showed up to smoke on the quad, some of them flying in from other states.

This year the University wants to end the ritual. They will close off the campus to anyone who isn't student or faculty, with violators facing trespassing charges. Police may also give out tickets to those smoking, though last year there were only 23 tickets and five arrests made for marijuana possesion.

The main way the campus is looking to prevent people from the smoke out is by fertilizing the quad with a strong fish-smelling fertilizer. 

"We're trying to do things to make it not a fun place to be," said University spokesman Bronson Hilliard to CNN.

Sixteen states have legalized the use of medical marijuana and Colorado and Washington are considering the legalization of the drug, according to the Associated Press.