It is celebrated during the first week of February in Piedmont, Italy. It is a huge food fight, consisting of men and women throwing oranges at each other.
The purpose is to pay tribute to a battle that happened in 1194 when the Mugnaia (miller's daughter) decapitated Raineri di Biandrate, the town Count. It may have begun with the tossing of beans, but somewhere along the way changed to oranges. Sounds like great fun.