"Ask the wild bee what the Druid knew"
-Scottish Highlander Proverb
The Druids had some serious mojo because the Roman Legion literally exterminated them in order to conquer Brittania. The Druids were the intellectual, spiritual, and political leaders of the Celts.
On or about the first of November the Druids
held their great autumn festival and lighted fires in honor of the Sun-god, in Thanksgiving for the harvest.
The Druids believed on the eve of this festival Saman, the lord of death, gathered all the wicked souls of the past 12 months will be condemned to inhabit the bodies of animals.
The Church in the 9th century endorsed 1st of November as the date of All Saints' Day and the 2 November All Souls Day.
Modern Halloween is a synthesis of the Druidic and Church Holy Days.