In 1922, an erupting volcano forced the people of Nejapa, El Salvador, to evacuate. As they were leaving, locals saw great balls of fire spewing out of the volcano and believed that their patron saint, San Jeronimo, was actually fighting the devil for them. So to honor this event, where their heroic saint saved the villagers from burning alive, everybody gets together once a year and burns each other alive.
"That volcano is a wussy little bitch!"
The city divides itself into two teams, then everybody wads up some old rags, dips them in kerosene for a month, sets them ablaze and hurls them at their neighbors, because apparently Jeronimo was the Patron Saint of Arson. Sure, the revelers mostly come equipped with water-soaked gloves, clothes and masks for safety, but you can only prepare for Armageddon if you know it's coming in the first place. If you just happen to stumble into the wrong village on the wrong day, however, then surprise!