I found this explanation of Halloween, elsewhere:
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Halloween is a totally idolatrous celebration, which originated as a Celtic holiday, “Samhain” (pronounced many different ways), named after their Avodah Zarah who was “Lord of the Dead and Prince of Darkness.”
Samhain supposedly took the “sun god” prisoner each year during the winter. On the day before the new year, which for the Celts was November 1st, Samhain called together all the dead people for a meeting. The dead people would take different forms, the really evil ones taking the form of a cat.
Of course, this was all very scary to the Celts, so they had their galachim, called “druids,” offer sacrifices that day. They made a holiday out of this, to honor both the sun god and Samhain, lasting three days, where people would parade down the street in animal skins and other costumes.
Then the Romans also had a holiday which, after many centuries, ended up being mixed in with Samhain Day. It’s called “Pomona Day,” named after their Avodah Zarah god of fruits.
Then about 1,200 years ago, the Roman Catholics declared November 1st a holiday, “All Hallows Day,” in honor of their saints. Later, they added another day to this, Nov. 2, called “All Souls Day,” in honor of dead people. The Christians dressed up as saints, angels, and devils.
They made these holidays in order to counteract Samhain’s Day (“Chukas Pagans” is against Christianity). But instead of counteracting it, people simply celebrated both the Christian and Celtic holidays at once.
The Halloween that exists today has a mixture of the customs of Samhain’s Day, Pomona Day, All Saints Day, and All Souls Day.
That’s what you’re celebrating on Halloween.