Der Spiegel had a story on the flour wars that take place in the Greek village of Galaxidi.
The day is known as "Clean Monday" (or Καθαρή Δευτέρα), though that is a bit misleading, given that villagers throw over a ton of colored flour at each other.
According to the article, "Preparations for the war -- also known as alevromoutzouromata or 'people throw flour at each other' -- are intense. Locals dye bag upon bag of flour with food coloring and paint their faces with charcoal.... And revellers, at least those who know what they are in for, bust out goggles to protect their eyes from the flying starch....
"The flour fight dates back to the very beginning of the 19th century... Villagers began celebrating Carnival in defiance of the Ottoman occupiers, painting their faces with ash and dancing in decorous circles, one for women, one for men. Now the fun is co-ed and the flour throwing non-discriminating."