Halloween is primarily credited with starting in Ireland and ancient Britain dating back approximately 2,000 years to the ancient Celtic festival of Samhain (pronounced sow-in).
The Celts celebrated their new year on November 1. This date marked the end of the harvest and the beginning of the dark, cold winter. Over centuries, the traditions of Samhain were gradually blended with Christian observances. In the 8th century, Pope Gregory III designated November 1 as a day to honor all saints, calling it All Saints' Day. That's how October 31st became All Saints (All Hallows) Eve.