It isn't harmless fun. Halloween comes from Samhain, the old Celtic festival held on the night of October 31 into November 1. It marked the end of summer and the start of winter, and the Celts believed it was the night when spirits walked the earth.
An old Irish tale, the Echtra Nerai, says of that night, "demons would appear on that night always," and that "the fairy-mounds of Erinn are always opened about Halloween."
Look at the customs that came out of it.
Bonfires were lit on Samhain eve to keep away fairies and witches.
People wore disguises so the evil spirits walking the earth would not recognize them. The Library of Congress says the Celts "donned disguises in order to confuse the spirits."
Fortune telling and divination were part of the night.
The dead were believed to come back and visit their old homes.
The Catholic Church didn't stop it. It put All Saints' Day on November 1 and All Souls' Day on November 2, with prayers for the dead, and the night before became All Hallows' Eve. Even the New Catholic Encyclopedia says the feast "may also have been Christian response to the Druid festival of the dead, Samhain."
Putting a church name on a pagan night doesn't make it holy. The Bible never mentions Halloween, All Saints' Day, or praying for the dead.
Praying to the dead is necromancy, reaching out to the dead, and God lists the necromancer right beside the witch and the wizard as an abomination.
What the Bible does say is plain. God calls divination, witchcraft, and consulting familiar spirits an abomination. He told His people not to learn the way of the heathen. He tells us to have no fellowship with the works of darkness, to come out from among them and be separate, and to abstain from all appearance of evil.
A night built around spirits, witches, and the dead is not something a child of God needs to dress up for.
Let your light shine instead.
Scripture References
Deuteronomy 18:10-12 (KJV) There shall not be found among you any one that maketh his son or his daughter to pass through the fire, or that useth divination, or an observer of times, or an enchanter, or a witch, Or a charmer, or a consulter with familiar spirits, or a wizard, or a necromancer. For all that do these things are an abomination unto the LORD: and because of these abominations the LORD thy God doth drive them out from before thee.
Jeremiah 10:2 (KJV) Thus saith the LORD, Learn not the way of the heathen, and be not dismayed at the signs of heaven; for the heathen are dismayed at them.
Ephesians 5:11 (KJV) And have no fellowship with the unfruitful works of darkness, but rather reprove them.
2 Corinthians 6:17 (KJV) Wherefore come out from among them, and be ye separate, saith the Lord, and touch not the unclean thing; and I will receive you,
1 Thessalonians 5:22 (KJV) Abstain from all appearance of evil.