The following is an email, which I received today.
Learn the traditions behind the holiday
Why do we celebrate halloween? Maybe it's because we've been celebrating it practically forever. Halloween is the modern version of one of the Western world's oldest holidays. It roots are in the ancient Celtic New Year's festival, Samhain, but it also includes elements of Christian and ancient Roman traditions. In pre-Christian Ireland, the Celts believed that on October 31, the spirits of the dead returned to earth. So each year on that night, the Celtic priests, or Druids, would build bonfires and offer sacrifices of crops and animals. The people dressed in costumes to disguise themselves from the spirits. This was probably the origin of our customs of dressing up for Halloween.
In the eight century A.D., the pope declared November 1 to be All Saints Day in the Christian world. In English, this holiday was also called All Hallows Day, which later became Halloween. Christainity had recently spread to the Celtic countries, and elements of the old festival of Samhain were incorporated into the celebrations of All Hallows Day and the following day, All Souls Day. On All Souls Day, people gave treats called "soul cakes" to beggars at their doors who promised to pray for the family's dead. Children eventually became the ones to beg for these treats, and this eventually gave rise to the "treat" part of trick-or-treating.
Irish immigrants fleeing their country's potato famine popularized Halloween in the United States in the 19th century. They brought with them the jack-o'lantern, which symbolized a mythical ghost doomed to walk the earth forever because he had angered both God and the devil. The new Americans found that pumpkins made even better jack-o-lanterns than the turnips and rutabagas they had used in Ireland.
American Halloween festivities have evolved over the years, from raucous events filled with pranks and ghost stories to quieter family fun, safe trick-or-tricking at malls, and neighborhood gatherings. But many of the holiday's ancient traditions make it the unique celebration that kids still love today.