I found this online at EWTN.
Here is a part of it and the link follows:
HALLOWEEN: ITS ORIGINS AND CELEBRATION
The celebration of Halloween has dual origins. The first is in a pre-Christian Celtic feast associated with the Celtic New Year. The second is in the Christian celebration of All Saints Day (Nov. 1st) and All Souls Day (Nov. 2). In the British Isles November 1st is called All Hallows, thus the evening before is All Hallows Eve.
The Celtic Feast
The ancient Celtic peoples who inhabited England, Wales, Scotland, Ireland and Brittany (NW France) celebrated their New Year’s Day on what would be November 1st on our calendar. Prior to their conversion to Catholicism these peoples practiced a pagan religion controlled by a priest class known as Druids. The Druids are most famous for the stone monument of Stonehenge and other astronomical calendars that remain in their former domains
You can find the rest of the article HERE