Śmigus-dyngus or lany poniedziałek. Wet Monday (Easter Monday) in Poland. Originally this Slavic custom meant whipping each other with birch twigs and throwing water at each other which used to symbolize the spring cleansing. This ritual was already in custom in 750, over 200 years before Poland officially adopted Christianity. Dyngus and Śmigus were twin pagan gods; the former representing water and the moist earth and the latter representing thunder and lightning .
On Easter Monday the water consumption in Poland shoots way up and this is due to an ancient custom that is still observed today, in villages and cities alike. It is called Smigus Dyngus or Wet Monday when people sprinkle each other with water on the day after Easter, or, in the case of young people, they don't sprinkle but douse each other with buckets of water.