Lany poniedziałek, called also Śmigus-dyngus is an old Polish tradition. People pour water on each other. It starts early in the morning. Everyone wants to get up early and be the first to wake all the family with water.
Small water toys, water guns, glasses of water or even bucket of water can be used!
Sources of the photos: my own, http://leantoys.pl/pol_pl_Pistolet-na-wode-4-kolory-do-wyboru-1230_1.jpg, https://imged.pl/4402/ogromny-dwulufowy-pistolet-na-wode-86cm-warszawa-384402.jpg, https://www.radio90.pl/files/2013/04/smigus.jpg
In some Polish towns you can meet big water battle if it is warm. Unfortunately, this year there is a lot of rain and the temerature is quite low. Still, in Kazimierz Dolny, a town in East of Poland at noon few cars of the fire brigade came to the market square and, as it happens every year, poured water on people and surrounding houses, to assure health, wealth and happiness. You can find some videos from the previous years here: http://www.kazimierzdolny.pl/tagi/lany_poniedzialek_w_kazimierzu/
So if you don’t want to get wet that day, you’d better take a water coat or an umbrella. 😉