What is this holiday known as Halloween anyway? According to History.com, it "originated with the ancient Celtic festival of Samhain, when people would light bonfires and wear costumes to ward off roaming ghosts. In the eighth century, Pope Gregory III designated November 1 as a time to honor all saints and martyrs; the holiday, All Saints’ Day, incorporated some of the traditions of Samhain. The evening before was known as All Hallows’ Eve and later Halloween."
Wikipedia says, it has "origins in the Roman feast of Pomona, the goddess of fruits and seeds, or in the festival of the dead called Parentalia, it is more typically linked to the Celtic festival of Samhain, whose original spelling was Samuin (pronounced sow-an or sow-in)".[1] The name of the festival historically kept by the Gaels and Celts in the British Isles which is derived from Old Irish and means roughly "summer's end".[1][2][3]
However, according to the Oxford Dictionary of English folk lore: "Certainly Samhain was a time for festive gatherings, and medieval Irish texts and later Irish, Welsh, and Scottish folklore use it as a setting for supernatural encounters, but there is no evidence that it was connected with the dead in pre-Christian times, or that pagan religious ceremonies were held." [4]
The Irish myths which mention Samhain were written in the 10th and 11th centuries by Christian monks. This is around 200 years after the Catholic church inaugurated All Saints Day and at least 400 year after Ireland became Christian."