Organizers say the aim is to "cover your neighbor in wine as quickly and completely as you can." After an early morning Mass is held, participants throw wine over each other while music plays until the 70,000 liters (18,500 gallons) of free wine run out.
Among the rules for those taking part: they must wear white, with a red sash; they may use wine-filled water pistols, garden sprayers or buckets; and at no point during the battle can they stop laughing and singing.
Thousands of people come away from Haro's vineyards and woodland soaked from head to toe. The event is held to celebrate St. Peter and the region's plentiful wine output. It ends with a big lunch. The festival draws mostly young visitors, from as far away as Australia.