Halloween
"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".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."
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.
The word Halloween is first attested in the 16th century and represents a Scottish variant of the fuller All-Hallows-Even ("evening"), that is, the night before All Hallows Day. All Saints' Day (in the Roman Catholic Church officially the Solemnity of All Saints and also called All Hallows or Hallowmas, in honor of all the saints, known and unknown." (WIKI)