Every October 31, children go out on something called "trick or treating" to ask complete strangers for candy while dressed-up with the appearance of evil and with their houses dressed up with the appearance of evil. Halloween doesn't even try to hide its satanic origins! And Paul correctly pointed-out that we should abstain from all appearance of evil. Yet Christians still find some way to try to justify continuing to celebrate this satanic day.
By the way, did you know that Paul was applying Deuteronomy 12 when he said to abstain from all appearance of evil? God himself commanded us not mimic the ways of the pagans, not even as a means of serving him, but said to keep his commandments without adding to or diminishing from them [Deuteronomy 12:32]. Here's an article giving a very brief summary about what the Celts did for their new year celebration, which was later hijacked by the Roman Catholic Church, being given the name "All Hallows Eve" assigned to October 31, and mimicking the Celtic witchcraft in their celebration to this day!
Actually, the article linked above gives a watered-down version of what the Celts did. A very watered-down version. There are still cultures around the world that practice Celtic witchcraft instead of "merely" mimicking it. And those who actually read the passages I linked (bonus points if you read more), you will know that God wants these pagan practices and their articles completely destroyed, leaving no trace that they even ever existed. He wants us to practice what he commanded, not what the pagans practiced. He has set us apart (made us holy) from the rest of the world to be an example to the world.
Let's take a look at some of the excuses that Christians make to continue to celebrate this holiday. Some who claim to be Christians outright lie about the origins of the various practices and claim that the pagans borrowed from Christianity. But even if that's true, what business does a Christian have in dressing as witches or demonic entities? What part of "abstain from all appearance of evil" [1 Thessalonians 5:22] do you not understand? What part of "do not add to God's commandments" do you not understand? Should God be pleased with us for dressing as those whom God said shall be put to death? Should God be pleased with us for honoring the rebellious angels by dressing as them? Should God be pleased at the glorification of death, which he says is the enemy [1 Corinthians 15:26]?
Some Christians claim to celebrate Halloween to make a mockery of Satan because he was defeated by Jesus dying on the cross and then rising from the dead. While I have no reason to question their sincerity, there are a couple a major problems with this, not counting the fact that it's a direct violation of Deuteronomy 12 and a violation of Paul's command. The first problem is that Jesus didn't die on October 31. He died on Aviv 14 (buried the following sundown) in 30 AD, which landed on Wednesday, April 3. He rose on Aviv 18 (at sundown), which landed on Sunday, April 7, but which would still be counted by our standards as Saturday (the Sabbath), April 6.
The second major problem is this: Who's mocking who? Is it the Christian making a mockery of demonic powers, or Satan making a mockery of Christians who have been tricked into doing things that are contrary to everything that God stands for? Satan's goal is to keep Christians away from God and he is the father of lies [John 8:44]. And what better way to do that than to trick us into believing that there's nothing wrong with doing things that God calls abominations? I think the answer to the question of "who's mocking who" is quite clear.
I'll leave you with this video on the origins of Halloween by Pastor Jim Staley.