This video is posted on many websites, almost always without explanation. A Google search finally yielded the following:
Clean Monday (March 10th) marks the first day of Orthodox Lent in Greece. Although there’s still an atmosphere of carnival on the streets, only “pure” food is eaten.Der Spiegel reports that the festival is also cleverly known as "alevromoutzouromata" or "people throw flour at each other."
Galaxidi, a city located 200 km/124 miles from Athens, is home to an annual flour war. The “war” is a long kept tradition which happens every year on the first day of Lent.
Given the messiness of the war, Clean Monday is not exactly a good word for it though. Every year, for the past 200 years, residents and visitors spend their day bombarding everyone with bags of colored flour. Because the dye in the flour leaves nasty stains, the old buildings in the town are covered in plastic sheets. No one seems to be spared so if you plan to visit Galaxidi this time of the year, just remember that getting colored flour all over yourself is not as idyllic as it might seem.
The flour fight dates back to the very beginning of the 19th century, according to the Greek tourism bureau. 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.It's not quite as spectacular as the "Rouketopolemos" (Greek Рουκετοπόλεμος, literally Rocket-War) held annually at Easter in the town of Vrontados.