From the 11th century, the Christian eve of All Saints is referred to in Gaelic as Féile na Marbh, or, the festival of the dead’.

The night of Samhain, in Irish, Oíche Samhna and Scots Gaelic, Oidhche Samhna, is one of the principal festivals of the Celtic calendar, and falls on the October 31. In modern Ireland and Scotland, the name by which Halloween is known in the Gaelic languages is still Oíche/Oidhche Samhna. It is still the custom in some areas to set a place for the dead at the Samhain feast, and to tell tales of the ancestors on that night,

Traditionally, Samhain was time to take stock of the herds and grain supplies, and decide which animals would need to be slaughtered in order for the people and livestock to survive the winter. This custom is still observed by many who farm and raise livestock because it is when meat will keep since the freeze has come and also since summer grass is gone and free foraging is no longer possible.

Bonfires played a large part in the festivities celebrated down through the last several centuries, and up through the present day in some rural areas of the Celtic nations and the diaspora. Villagers were said to have cast the bones of the slaughtered cattle upon the flames. In the pre-Christian Gaelic world, cattle were the primary unit of currency and the center of agricultural and pastoral life. Samhain was the traditional time for slaughter, for preparing stores of meat and grain to last through the coming winter.

http://en.wikipedia.org/wiki/Samhain