I dressed the Pups up for Halloween, but mainly so we could have fun handing out candy to kids, which I like to do every year. What I didn't do was dress us my dog in order to celebrate Satan, and I'm mostly sure that the 6 y/o boy dressed as a T-rex wasn't too focused on that, either. The girl dressed as a Frozen princess who took TWO handfuls of candy may have had a little bit of an unclean spi—just kidding, she was from the local congregation. :D
That said, some associate Halloween with Satan, as they see a connection between it and ancient druidic, pagan practices and they believe this strongly enough that it bothers their conscience. That is fine! Although my conscience is clean, others may be bothered. In 1 Corinthians 8, some could eat food sacrificed to idols all day long without a care, but others were spiritually damaged by it. Some now can't see the candy apart from a connection to paganism, and some can.
Likewise, Romans 14 addresses these things extensively. In part it says, " One person regards one day above another, another regards every day alike. Each person must be fully convinced in his own mind [...] But you, why do you judge your brother? Or you again, why do you regard your brother with contempt? [...]This faith which you have, keep between yourself and God. Happy is he who does not condemn himself in what he approves. But he who doubts is condemned if he eats, because his eating is not from faith; and whatever is not from faith is sin."
Like Christmas and Easter, if it bugs you, certainly don't partake. And those of us who are not bugged by it should not push it on those who are, thus making them stumble.
"For the entire Law is fulfilled in in this one word: "You shall love your neighbor as yourself."—Gal 5:14