Are people who participate in Halloween appropriating ancient Celtic culture?

2. Ghosts

Celtic people believed that during the festival Samhain, which marked the transition to the new year at the end of the harvest and beginning of the winter, spirits walked the Earth. Later, the introduction of All Souls Day on November 2 by Christian missionaries perpetuated the idea of a mingling between the living and the dead around that time.

3. Costumes

With all these ghosts wandering around the Earth during Samhain, the Celts had to get creative to avoid being terrorized by evil spirits. To fake out the ghosts, people would don disguises so they would be mistaken for spirits themselves and left alone.

4. Trick-or-Treating

Everyone can agree that free candy is awesome. Beyond that, there’s lots of debate around the origins of trick-or-treating. One theory proposes that during Samhain, Celtic people would leave out food to placate the souls and ghosts and spirits traveling the earth that night. Eventually, people began dressing up as these otherworldly beings in exchange for similar offerings of food and drink.

This post was edited by Mr. Bromide at October 27, 2017 2:16 PM MDT

Halloween commemorates the start of Noah's flood. Lots of specially trained smart people have looked hard for evidence of a world wide flood. They found two. Neither of them was Noah's flood of 2349 BC. Nevertheless, people all over the world recorded a flood that year and they all agreed on all the details. You might be surprised to learn what actually happened. This is long, but it's all there: https://www.saturniancosmology.org/

If you don't care to read the whole book, here is the section about Noah:
https://www.saturniancosmology.org/noah.php

Some of that sounds familiar.
[...
Perhaps in no other “Christianized” celebration does Satan so blatantly honor himself and memorialize his war dead. The writer J. Garnier suggests that celebrations of suffering and death can be traced back to the ancient destruction of all of his human followers, as well as the hybrid sons of fallen angels, at the time of the Flood. Cultures the world over have festivals for the dead, “held by all on or about the very day on which, according to the Mosaic account, the Deluge took place, viz., the seventeenth day of the second month—the month nearly corresponding with our November.”—The Worship of the Dead, by J. Garnier.
The Druids were no exception. On October 31, Samhain was said to release the spirits of the dead to mingle with the living. Druids roamed the streets with lanterns, and on coming to a house, they demanded money as an offering for Satan.
Halloween is a major satanic ritual day. “It’s a religious holiday for the underworld, with satanists performing sacrifices and witches quietly celebrating with prayer circles or meals for the dead,” according to a USA Today article. It quoted Washington witch Bryan Jordan as saying, “[Christians] don’t realize it, but they’re celebrating our holiday with us. . . . We like it.”
...]

Yup, pretty much. Even tried to give it a godly veneer by calling it a 'Festival to Jehovah'. Kind of like including Christ in some Pagan festivals that they tried to mix in something godly.

Sometimes I wonder if the world would have been better off if Constantine had left the Roman Empire a polytheistic region, rather than insisting on changing it to a monotheistic one.

I mean, for thousands of years they just absorbed different deity belief systems - rather than trying to stamp them out.

Just part of the apostasy foretold in the Bible. Part of the weeds, so to speak. Even that 3-n-1 God that he decided on was borrowed from other religions.