Halloween is All Hallows’ Eve... Hallows being Saints...a/k/a All Saints’ Eve, the date before All Saints’ Day and the beginning of a liturgical period set aside for remembering the saints and martyrs and the Christian departed ...a Christian celebration predating pagan records. As it happens, All Saints’ Day is also Reformation Day.
Others just try to usurp it, claiming it has something to do with Celtic harvest festivals, and a Gaelic celebration, which it does not, other than the coincidence of taking place in autumn.
The association of pagan lore with All Saints’ Eve is recent and due to secular tendencies to try to make a buck on a holy day.
I have relatives and friends who are cops, agents, and detectives. Trust me: Satanists and all their agents treat Halloween as a “sacred” day for the evil one. There are stories that you would not believe.