Halloween is an ancient holiday and my personal favorite one of them all. It began from Celtic and Gaelic harvest festivals.
Interesting, I did not know that!  Did they dress up in costumes back then as well?
Halloween
Halloween or Hallowe'en (a contraction of Hallows' Even or Hallows' Evening), also known as Allhalloween, All Hallows' Eve, or All Saints' Eve, is a celebration observed in several countries on 31 October, the eve of the Western Christian feast of All Hallows' Day. It begins the three-day observance of Allhallowtide, the time in the liturgical year dedicated to remembering the dead, including saints (hallows), martyrs, and all the faithful departed.
It is widely believed that
many Halloween traditions originated from ancient Celtic harvest festivals, particularly the Gaelic festival Samhain; that such festivals may have had pagan roots; and that Samhain itself was Christianized as Halloween by the early Church. Some believe, however, that Halloween began solely as a Christian holiday, separate from ancient festivals like Samhain.
Halloween activities include trick-or-treating (or the related guising and souling), attending Halloween costume parties, carving pumpkins into jack-o'-lanterns, lighting bonfires, apple bobbing, divination games, playing pranks