Don't believe me? Look up something like "halloween is satanic" and google says, "2,340,000 results (0.08 seconds)". Most of the top searches are from idiots like the people at demonbuster.com or exposing satanism.org or the jeremiahproject.com, etc. They darkly warn that even things like "bobbing for apples" were used in pagan fertility rights. Yep, most single adults who are not losers go to halloween parties and have a few drinks and try to get lucky. Naturally, religious fanatics disapprove of such things.
They like to quote passages like 1 Thess 5:22, which the Queen James (yes, he was gay) Bible renders as "Abstain from all appearance of evil", because they want to twist the word appearance to mean something like a costume. However, modern translations, like the NIV render this simply, "reject every kind of evil". Indeed the context makes clear that they are saying nothing about how someone physically appears. Indeed the first few verses before say, "Do not quench the Spirit. Do not treat prophecies with contempt but test them all; hold on to what is good". Right after this it says, "comma reject every kind of evil". So, no, it's not saying that you can't wear a scary costume.
Most of these anti-halloween propaganda pages play fast and loose with Bible verses, and are incredibly poorly researched. They declare, without the slightest evidence, that Halloween is the "Devil's Birthday". However, presumably the Devil was created, not born. Think about it. If the Devil had a birthday then the Devil had a mother and a father. By many traditional, extra-biblical accounts the Devil or haSatan (the enemy) is a fallen angel. That would mean that angels were having sex in Heaven and conceiving children.
So why have we never heard of this mother and father? Yet the Bible says that angels are essentially spirit beings who sometimes take physical form, as when Hebrews 1:14 says, "Are not all angels ministering spirits". This is yet one more example of the mindless nature of the anti-halloween, anti-fun fanaticism of EVILgelical Christians.