1 Corinthians 9:19-23 "Though I am free and belong to no one, I have made myself a slave to everyone, to win as many as possible. To the Jews I became like a Jew, to win the Jews. To those under the law I became like one under the law (though I myself am not under the law), so as to win those under the law. To those not having the law I became like one not having the law (though I am not free from God’s law but am under Christ’s law), so as to win those not having the law. To the weak I became weak, to win the weak. I have become all things to all people so that by all possible means I might save some. I do all this for the sake of the gospel, that I may share in its blessings."
Christians should celebrate Halloween for the opportunity to "spend time/build relationships" with those who don't know Christ. Yes, Halloween can be traced back to pagan practices and festivals... who cares! I believe in the power of God to redeem things meant for evil, and use them for good.
P.S. A dead, carved vegetable only has evil powers, if you think it does.