has written a very thoughtful and helpful post,"Why Christians Should Celebrate Halloween," which explores the history of the Church's interaction with the ancient European autumn harvest/death festivals. For both the Romans and the Celts, autumn marked a time to celebrate the harvest, and prepare for the oncoming long darkness of winter; the former by enjoying the abundance of food, the latter by offering sacrifices, performing incantations and, in some cases, dressing up to appease or ward off the evil spirits of death and darkness.
The Church stepped into this milieu with a message of hope: the God who had created all, and was therefore the source of all good things, had come into the world and defeated death! The darkness no longer need be feared for Christ's light had conquered it!