La Tomatina.

         I know i'm wouldn't be alone in saying that a massive tomato food fight would be AWESOME! This is probably one celebration above all the others that I wished we celebrated it in the U.S. or at least a smaller version! This kind of thing is like the food fight that people - as kids - always talk about and joke about but it never seemed to happen no matter how tempting it was to just toss a unwanted piece of food in the other direction.This tradition isn't new by any means but it isn't that old either, however it has the attention of the whole world and thousands of foreigners come to participate along with the traditional spanish people.
    
      "La Tomatina" a tomato fight in Buñol near Valencia happens every year on the last Wednesday in August but the clebration starts a few days earlier. The most famous part of the festival is the tomato fight which takes place between 11am and 1pm on that day.The event has become one of the most famous of Spain's summer festivals with thousands of people flocking to this little town for this rather crazy event. The origins of it aren't clear with several theories explaining how it came to be. However, the most likely theory says that the explanation dates back to 1945 when a parade of enormous figures with big heads was passing through the streets of Buñol. Apparently some kids tried to join in the parade and knocked over one of the giants who got to his feet and started swinging out at everyone around him. The kids fought back by grabbing tomatoes and throwing them at him until police stopped them. This is one of the coolest traditions of Spain if not the world, I really hope that some time in my life I can make it there during these days and that I get to participate in this awesomely hectic event that really just seems like a lot of messy fun! It really just seems like a good time to me.

Site: http://www.spanish-fiestas.com/spanish-festivals/la-tomatina-tomato-battle-bunyol.htm