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.
"We're telling people they'll have a lot fewer headaches if they stay in Denver."
Police may also give out tickets to those smoking marijuana on campus, though in previous years few tickets have been given out. Last year, campus police handed out 23 tickets and made five arrests for marijuana possession.
CU police department spokesman Ryan Huff said the smoke-outs have become too dangerous.
"It's hard to keep track of that many people high on marijuana," he said.
Why these marijuana advocates congregate each year on April 20 is a bit of a mystery steeped in the hazy lore of the American pot culture. Some have said 420 is a local police code for someone smoking marijuana. Others have said it refers to the number of active chemicals in marijuana.
Another theory holds that 420 is a nod to Bob Dylan and his 1966 song "Rainy Day Women #12 & 35" which contains the lyric, "Everybody must get stoned." Multiply 12 by 35 and you get 420.
Conrad said the leading theory traces the beginning of 420 to the early 1970s when a group of students at San Rafael High School in California would meet at 4:20 each afternoon to smoke marijuana.
"It (was) the time between when classes were over and their parents came home," he said. "The number 420 signifies that somebody is smoking marijuana right now and it becomes a clarion call for others to join."
According to Conrad, the story goes that the San Rafael teens named themselves "the Waldos" and would meet very day at 4:20 p.m. near a statue of Louis Pasteur to smoke joints.
A website that claims to be associated with the Waldos declares them to be "The Founding Fathers of 420," although it doesn't appear to be current. Attempts to reach the owners of the site were unsuccessful.
No matter what the origin, the number now signifies the pro-marijuana culture: Craiglist ads for roommates, or for even so-called "casual encounters," often note "420 Friendly." An episode of the animated TV show Family Guy called "420" involves a plot in which marijuana becomes legalized and 420 Magazine is devoted to marijuana culture. The 2003 California State Senate bill that became the law that established medical marijuana in the state was called SB420.
What began is a secret code word between stoners has gone mainstream.
"It's a form of shorthand essentially. As far as being a secret code, it's one of the least secret," explained Conrad with a laugh.
9/26/15 - Titus Andronicus @ Roxy // 10/20/15 - Blur @ Hollywood Bowl // 11/12/15 - Desaparacidos @ Teragram Ballroom // 11/14/15 - TV On The Radio @ Theater at Ace Hotel
11/18/15 - No Age @ Echo // 11/21/15 - Blind Guardian @ City National Grove of Anaheim // 12/12/15 - HEALTH @ Teragram Ballroom // 2/13/15 - Black Sabbath @ Mandalay Bay Events Center
October 5 - Danzig/Superjoint
October 8 - Claude VonStroke/Disclosure
October 14 - Ghost
November 4 - Puscifer
November 7 - FFF/Mayhem/Watain/Rotting Christ
November 14 - Deafheaven
November 22 - GWAR
December 13 - Bell Witch
February 13 - Black Sabbath
March 24 - David Gilmour
I'm neither alone, jealous, or stoned, but i've been obsessed with this song since I first heard it. And, it probably belongs here.
Don't forget, it's Barney Gumble's Birthday too.
07/24 Social Studies @ GAMH (?)
07/28 Yumi Zoumi @ The Rickshaw Stop
07/31 Ratatat @ The Regency
08/07-08/09 OSL @ Golden Gate Park
08/17 Cayucas @ The Independent
08/26 MSTRKRFT @ The Mezzanine
08/28 Booka Shade @ The Mezzanine
09/18-9/20 TBD Fest @ Sacramento
09/23 Say Lou Lou @ The Independent
10/02 The Tragically Hip @ The Masonic
10/03 Tove Lo @ The Fox Theatre (?)
10/10 Beirut @ The Greek Theatre
10/14 AlunaGeorge @ The Mezzanine (?)