Easter Monday is the second day of polish feast after Easter Sunday. In Polish it is called: Lany Poniedziałek or Śmigus-Dyngus. This day is a public holiday.
That day in polish tradition boys and young men threw water on young women and girls who they thought were attractive and loved ones.
Some years ago, especially in big cities it meant danger of being soak to the skin by teenagers hunting with big basket in the streets on virtually everyone.
The polish governments and officials have been fighting with such a tradition for some last years, and trying to put this custom in moderate way: so you can use water but try to not exaggerate with it and not harm any one.
All in all it is not a good day for taking a walk and when it rains we say: this is a really Lany Poniedziałek.