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Its true name is Samhain: it is the feast of the Dead in the Celtic Calendar. On this night the veils between the worlds are lowered and not only can a dedicated person seek advice from the Other-worlds but the dead ancestors can reach out to the living.
Samhain marks several things. As with all Celtic pagan feasts it marks a point on the wheel of the year, in this case the end of the year, and beginning of the New Year. This date, obviously, was a great occasion in Celtic society. In this sense alone, one would the progress of the year; the death of the world which will (they all devoutly hoped) be reborn in Spring. This view of the world was enshrined in sagas and in folktales, in stories about the mother god giving birth to the son, who dies and then is reborn (concept carried on in many cultures and for many generations! ) or in the two lovers who are separated by foul means, by death or magic.
Samhain was the period of the year when the livestock which would not make it through the winter was marked out and slaughtered, to be feasted on and to be dried out as provision for the long dark months ahead. This, coupled with the sense of the world going underground for the winter, led to this feast being a feast uniquely concerned with death and the spirit world. At this time, the veils between the world of living and dead were felt to be very flimsy and our ancestors instinctively realized that the spirits, and ancestors, were close at hand.