This is the best of the current pieces I have come across on the web about the origins of Halloween. It's from the Sierra Star.
History and Hauntings
By Cathie Campbell – Special to the Sierra Star
(Updated Friday, October 28, 2005, 9:00 AM)
Beware of “bumps” in the night as corn stalks rustle in the fall breeze and jack-o-lanterns grin with eerie faces that cast flickering shadows. The streets will soon be filled with costumed children ringing doorbells for treats. It's Halloween time, but how and where did it all start?
According to The History Channel, Halloween's origins date back to the ancient Celtic festival of Samhain (pronounced sow-in).
About 2,000 years ago, in an area now known as Ireland, the Celts celebrated their new year on Nov. 1. It not only marked the end of summer and harvests, it signaled the beginning of the cold, dark winter, which was often associated with death.
Celts believed that on the night before the new year, the boundary between the worlds of the living and the dead became blurred. On the night of Oct. 31, when they celebrated Samhain, it was commonly believed that the ghosts of the dead came back to earth.
In addition to wreaking havoc and damaging crops, Celts thought the presence of these spirits made it easier for the Druids, or Celtic priests, to make predictions about the future.
For a people entirely dependent on the volatile natural world, these prophecies were an important source of comfort and direction during the long, dark winter.
To commemorate the event, Druids built huge sacred bonfires, where the people gathered to burn crops and animals as sacrifices to the Celtic deities.
During the celebration, the Celts wore costumes, normally made from animal heads and skins, and attempted to tell each other's fortunes.
When the celebration was over, they re-lit their hearth fires, which they had extinguished earlier that evening, from the sacred bonfire to help protect them during the coming winter.
By A.D. 43, Romans had conquered the majority of Celtic territory. In the course of the 400 years when they ruled the Celtic lands, two festivals of Roman origin were combined with the traditional Celtic celebration of Samhain.
The first was Feralia, a day in late October when the Romans traditionally commemorated the passing of the dead.
The second was a day to honor Pomona, the Roman goddess of fruit and trees. The symbol of Pomona is the apple and the incorporation of this celebration into Samhain probably explains the tradition of “bobbing” for apples that is practiced today on Halloween.
By the 800s, the influence of Christianity had spread into Celtic lands. In the seventh century, Pope Boniface IV designated Nov. 1 All Saints' Day, a time to honor saints and martyrs.
It is widely believed today that the pope was attempting to replace the Celtic festival of the dead with a related, but church-sanctioned holiday.
The celebration was also called All-hallows or All-hallowmas (from Middle English Alholowmesse meaning All Saints' Day) and the night before it, the night of Samhain, began to be called All-hallows Eve and, eventually, Halloween.
Even later, in A.D. 1000, the church would make Nov. 2 All Souls' Day, a day to honor the dead. It was celebrated similarly to Samhain, with big bonfires, parades, and dressing up in costumes as saints, angels, and devils.
Together, the celebrations, the eve of All Saints' and All Souls', were called Hallowmas.