By Jim Spellman, CNN
Boulder, Colorado (CNN) - When the clock strikes 4:20 p.m. on April 20, or 4/20, marijuana fans will come out of the shadows to proudly smoke pot in parks and on college campuses across the country. The number 420 has become synonymous with all things marijuana, but exactly why is less clear.
Whatever the number's origin, "420" events across the country have become opportunities to advocate the legalization of marijuana. The expansion of medical marijuana in California, Colorado and other states is making efforts to legalize marijuana more mainstream and making more people comfortable coming out and smoking pot in public, according to Chris Conrad, curator of the Oaksterdam Cannabis Museum in Oakland, California.
One of the biggest pro-pot rallies is the annual smokeout on the campus of the University of Colorado at Boulder. The rally has taken place for about a decade and, in recent years, attendance has grown, according to university spokesman Bronson Hilliard. Last year, more than 10,000 people showed up to light up on the campus' Norlin quad.
"People fly in from around the country to participate," Hilliard explained. "We don't understand why they have to come to (this) campus."
This year the university is attempting to put an end to the annual ritual. They will shut down the campus to everyone except faculty, staff and students. Violators could face trespassing charges.
To discourage students from lighting up, the university has decided to fertilize the grassy area of Norlin quad on Friday.
"We're trying to do things to make it not a fun place to be," said Hilliard. "We are using a fish-based fertilizer. It is a rather foul-smelling emulsifier.