In the Christian tradition there has been a custom of "dressing up" since the early Christians celebrated the "breaking of bread" and then continued the celebration with a party.
By dressing in the costumes of death, the devil, or some other demonic image, we become and recognize our own death, our own devil, our own demons. We recognize rather than deny its existence and therefore, paradoxically, deny its power over us. If I acknowledge my anger and give it expression it will not swell up inside me as bottled rage that will someday explode into destructiveness that "possesses" me and victimizes others. If I can put on the mask of death for a night, I become less fearful of death the other nights of the year. If I journey into the darkness, I will have confidence in the coming of light.
Within the Christian tradition a Halloween costume party becomes a sharing of the communion of saints on hallowed eve. Stripped of commercialization and superstition, the heart of Samhain, a Celtic summer’s end festival is profoundly Christian. It is a recognition of the genuine presence of darkness that continually challenges the light. In November we are aware of the shortening of the days and the waning of the power of light and life. This is not only a physical reality, but a psychological and spiritual metaphor. The gloom of winter is setting in, and we need to be prepared to cope with it.
Liturgically, All Saints/All Souls is an important preparation for Advent: it is a recognition of the coming of the darkness that in its turn will be overcome by the coming of the LIGHT. Simply put, if there is no darkness there is no need for Advent and Christmas; but if there is darkness, it must first be recognized and accepted before it can be challenged.
On the other hand Hallowed Eve is also a celebration of the whole Christian community's ability to overcome death. Rooted in the church's experience of the Risen Christ, the whole communion of saints - living and dead - celebrate their power over darkness. During the Halloween season, the communion of saints celebrates its victory over the season of death and the time of darkness. During the two Christian feasts of All Saints and All Souls we continue to celebrate the triumph over darkness and the communion of saints.
For more information see Modern Liturgy, Volume 16 Number 8