In Ireland, tonight is called Oiche Samhain, or night of Samhain. The Celtic Festival of Samhain, marks the end of the old year and beginning of the new year. It also marked the end of the harvest and the start of the dark half of the year.
There is the idea that being a time of transition the boundaries between the underworld and the human world are not secure and so all kinds of eerie souls, spirits, ghosts and fairies can move freely in the human world, many of whom would be up to no good!
There are many traditions associated with the desire to ward off these unwanted spirits or indeed to welcome your own deceased kin. Bonfires would be lit and people would wear masks to confuse the spirits. One nice story I heard during the week was that people in one area would leave a basin of water and a towel by the doorway so that passing spirits could wash!