From Wikipedia:
Śmigus-dyngus (also known as lany poniedziałek, meaning "Wet Monday", Ukrainian: Поливаний понеділок) is a celebration held on Easter Monday in Poland and Ukraine. It is also observed by Polish diaspora communities, particularly among Polish Americans, who call it Dyngus Day.
Similar celebrations are held in the Czech Republic and Slovakia (Oblévačka in Czech, Oblievačka in Slovak, both meaning "Watering") and in Hungary, where it is known as Vízbevető or Water Plunge Monday.
Traditionally, boys throw water over girls and spank them with pussy willow branches on Easter Monday, and girls do the same to boys on Easter Tuesday.
This is accompanied by a number of other rituals, such as making verse declarations and holding door-to-door processions, in some regions involving boys dressed as bears.
The origins of the celebration are uncertain, but it may date to pagan times (before 1000 AD); it is described in writing as early as the 15th century. It continues to be observed in Central Europe, and also in the United States, where certain patriotic American elements have been added to the traditional Polish ones.