Known as an ancient tradition, in central-European countries like Poland and the Czech Republic, Wet Monday appears to be very popular in Ukraine, as well. It takes place on the second day of Easter
Wet Monday started out, in Poland,as a pagan custom that symbolize cleansing, with the coming of Spring. When Christianity became the main religion, Wet Monday was adopted as a Christian ritual, related to cleansing souls of sins. The truth is people loved this tradition so much, they found a way to keep it, by associating it with religion.
Click for more details Here