Wikipedia :

Easter Monday is the day after Easter Sunday and is celebrated as a holiday in some largely Christian cultures, especially Catholic.

Formerly, the post-Easter festivities involved a week of secular celebration, but this was reduced to one day in the 19th century. Events include egg rolling competitions and, in predominantly Catholic countries, dousing other people with water which, at one time, had been holy water blessed the day before at Easter Sunday Mass and carried home to bless the house and food. Many others believe the real origin of Wet Monday is due to the large quanity of Vodka and Rum typically consumed by a typical Polish or Eastern European person especially over a holiday weekend. The dousing was used to wake a hung over person in order to wake them so they could return to work.