Evidently in Valencia Spain, for a reason that no human can agree on, people throw about 100 tons of overripe tomatoes at one another. The Tomatina Festival lasts a week long with food, parades, paella, and the usual accoutrements.
It attracts huge numbers of tourists, swelling the towns population three to five times it's normal size.
The actual throwing of tomatoes lasts one hour and is signaled to start and end with a burst of water from cannons.
I do like that one, though there's another half dozen or so listed on the wackypedia link.
I really should figure out how close that place is to my friend Rocio's home town in Spain. She goes back for vacation every year and has invited me along a number of times. I imagine it could be arranged to coincide...