Śmigus Dyngus, also called Lany Poniedziałek (Wet Monday) in Poland is celebrated on Easter Monday. It has its roots in old Slavic traditions of throwing water on people in rites meaning to purify them for the arrival of spring.
Vintage Easter postcard showing Śmigus Dyngus in a village of the Kraków region, 1938, via historiarabki.blogspot.com