Dyngus Day or Wet Monday (Polish Śmigus-Dyngus or lany poniedziałek) is the name for Easter Monday in Poland. In the Czech Republic it is called velikonoční pondělí or pomlázka. In Slovakia veľkonočný pondelok (Easter Monday) is called Šibačka/Polievačka or Oblievačka too. All countries practice a unique custom on this day.

In Poland, Slovakia, and the Czech Republic^[3] traditionally, early in the morning boys awake girls by pouring a bucket of water on their head and striking them about the legs with long thin twigs or switches made from willow, birch or decorated tree branches; however, the earliest documented^{[citation needed]} records of Dyngus Day in Poland are from the 15th century, almost half a millennium after Poland adopted Christianity.

Benedykt Chmielowski in Nowe Ateny cite after "Carolo Berthold" that this ritual was already in custom in 750, 250 years before Poland officially adopted Christianity.

One theory is that Dyngus originates from the baptism on Easter Monday of Mieszko I (Duke of the Polans, c. 935 - 992) in 966 AD, uniting all of Poland under the banner of Christianity. Dualism and "twins" are featured in Slavic pre-Christian paganism. Dyngus and Śmigus were twin pagan gods; the former representing water and the moist earth (Dyngus from din gus - thin soup or dingen - nature); and the later representing thunder and lightning (Śmigus from śmigać or to make a whooshing sound). In this theory, the water tradition is the transformation of the pagan water god into the Christian baptism. The custom of pouring water was an ancient spring rite of cleansing, purification, and fertility. It is alleged that the pagan Poles bickered with nature/Dyngus by means of pouring water and switching with willows to make themselves pure and worthy of the coming year. Others have suggested that the striking tradition is the transformation of the ritual "slap" of Christian confirmation. However, still others suggest that the Śmigus tradition is actually simply a youthful recapitulation of a Good Friday Polish tradition, in which parents wake their children with switches from twigs, saying the words of a Lent prayer "God's wounds" - "Boże rany".

Early, the Dyngus custom was clearly differentiated from śmigus: dyngus was the exchange of gifts (usually eggs, often decorated - pisanka pl. pisanki), under the threat of water splashing if one party did not have any eggs ready, while Śmigus referred to the striking.

Later the focus shifted to the courting aspect of the ritual, and young unmarried girls were the only acceptable targets. A boy would sneak into the bedroom of the girl he fancied and awaken her by drenching her with multiple buckets of water. Politics played an important role in proceedings, and often the boy would get access to the house only by arrangement with the girl's mother.

Throughout the day, girls would find themselves the victims of drenchings and leg-whippings, and a daughter who was not targeted for such activities was generally considered to be unatractive in this very coupling-oriented environment.

Most recently, the tradition has changed to become fully water-focused, and the śmigus part is almost forgotten. It is quite common for girls to attack boys just as fiercely as the boys traditionally attacked the girls. With much of Poland's population residing in tall apartment buildings, high balconies are favorite hiding places for young people who gleefully empty full buckets of water onto randomly selected passers-by.

Another related custom, unique to Poland is that of sprinkling bowls (garce) of ashes on people (starts men on women) or houses, celebrated a few weeks earlier at the "półpoście." This custom is almost forgotten, but still practiced in the area around borders of Mazuria and Masovia

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