From the Wikipedia article, Dyngus Day:
Dyngus Day or Wet Monday (Polish Śmigus-dyngus, lany poniedziałek or Oblewania) is the name for Easter Monday in Poland. In the Czech Republic it is called Velikonocni Pondeli or Pomlázka.
Both countries practice a peculiar custom on this day. Traditionally, boys will awaken girls early in the morning and douse them with water and strike them about the legs with long thin twigs made from willow, birch or decorated tree branches (palmy wielkanocne). This practice is possibly connected to a pre-Christian, pagan fertility rite, although the earliest documented records of Dyngus Day in Poland are from 15th century, almost half a millennium after Poland adopted Christianity.
Most recently, the tradition has changed to become entirely water-focused, and the Śmigus part is almost forgotten. It is quite common for girls to attack boys just as fiercely as the boys traditionally attacked the girls. With much of Poland's population residing in tall apartment buildings, high balconies are favourite hiding places for young people who gleefully empty entire buckets of water onto randomly selected passers-by.
Here in the U.S., the spanking element has pretty much disappeared, and the liquid most common on Wet Monday is beer. Dyngus Day is mainly an occasion for eating sausage and quaffing brew. In South Bend, it's also the traditional kickoff for political campaigns. In 1968, the last time the Indiana Democratic presidential primary had any significance, Robert Kennedy was at the West Side Democratic Club on Dyngus Day. He won South Bend and Indiana handily.
Tonight, I expect Joe Donnelly to be at that same club. He's running against Chris Chocola, the plutocratic Republican congressman from Indiana's Second District. Two years ago Donnelly, a virtual unknown and without support form the national Democratic party, lost to Chocola by eight percentage points. This year, Democrats are taking the Second District seriously. MoveOn.org has been running ads against Chocola. Donnelly is a fairly conservative Democrat, like former Congressman and 9/11 Commission member Tim Roemer, who represented what was then the Third District from 1991 to 2003. But compared to Chocola, the guy's a godsend.