The Eve of All Hallows (All Saints') Day - October 31st. Coincides with the Celtic festival of Samhain - the end of Summer and Celtic New Year's Eve. It was a time of year when the veil between the worlds was thin and the spirits of the departed would return and mingle with the living.
The Romans added the festival to honour the Goddess of Fruit and Trees which probably contributed to aspects of Wassail and apple bobbing. It also coincided with Feralia - the Roman celebration for the dead.
Christianity made the following day a celebration of all Saints (all those who were Hallowed and didn't have ther own day) and the day after that, a day for All Souls now departed (A day for the dead).
Halloween was one of the times for Mischief Night.
Nowadays it is a horribly commercialised and Americanised 'Trick or Treat' night. An increasing number of people display notices provided by the Police advising participants that the household will not be taking part.