The story is simple: On a hot summer day in 1979, one guy told his buddies that he would buy free drinks for anyone willing to run outside and moon the passing train. Several took him up on his offer. A free, cold beer on a sunny California afternoon, it seems, is worth exposing yourself for. The tradition stuck.

Today, there is nobody volunteering to buy free drinks, but on the second Saturday of July every year, thousands of people gather along the chain link fence that runs adjacent to the Amtrak train tracks outside of Mugs Away Saloon in Orange County, California, to show their butts to passersby. Reportedly, trains passing the “moon show” slow down between Irvine and San Juan Capistrano stations.

Planned a year in advance with an official website, the event draws people from all over the country and has turned into a festival of sorts. Throughout the day, bands perform in Mugs and groups pack picnics.

In 2009, the Laguna Niguel City Council unanimously adopted two new ordinances against public consumption of alcohol and public urination in an attempt to discourage the annual event. “Thousands of rowdy partiers from out of town have forced us to take this action,” the mayor at the time said. “This event happened for many years without problems, but it has now gotten out of control.” More than ten thousand people had turned out the year before and residents of the area complained about public nudity and drunken tomfoolery.