As millions of children and adults participate in the fun of Halloween on the night of October 31st, few are aware of its ancient Celtic roots in the Samhain festival.
In Celtic Ireland about 2,000 years ago, Samhain was the division of the year between the lighter half (summer) and the darker half (winter). At Samhain the division between this world and the otherworld was at its thinnest, allowing spirits to pass through.
Christianity incorporated the honouring of the dead into the Christian calendar with All Saints (All Hallows) on November 1st, followed by All Souls on November 2nd. The wearing of costumes and masks to ward off harmful spirits survived as Halloween customs. The Irish emigrated to America in great numbers during the 19th century especially around the time of famine in Ireland during the 1840’s. The Irish carried their Halloween traditions to America, where today it is one of the major holidays of the year. Through time other traditions have blended into Halloween, for example the American harvest time tradition of carving pumpkins.
( http://www.newgrange.com/samhain.htm )