Some suggest the custom symbolizes baptism, specifically the Easter Monday baptism of the legendary Prince Mieszko (c. 935-992), who united all Poland under Christianity. Others say it is rooted in an ancient custom of throwing someone into the river to ward off evil spirits.
Prince Mieszko was Poland's first ruler. He was the duke of Polonia (which later became the country's name). As a young man, Mieszko was known for waging battle. He extended Poland to the Baltic Sea and into Bohemia. It's thought he converted in 966 because his first wife was a Christian. But he may have also converted to avoid having Poland incorporated into the Holy Roman Empire. Mieszko built a church dedicated to St. George, and the first Polish cathedral in Poznan.
- Little White Book, Diocese of Saginaw