Going to a Halloween party and hanging with friends is not "practicing or celebrating". I'm sure Christ will not pull the lever and send you down the Hell chute, because that's just plain stupid. He knows each of us, and why we do what we do. If you go out and sacrifice small animals on an altar, that's bad. If you participate in an Americanized, water-down, tacky-ass event like the one most will attend tonight, you are not sinning. You are living your life and hanging with friends.

As a "christian", I would not go to parties.
But as a disciple of Christ, I go anywhere I choose. I have the discernment necessary to make decisions, and those decisions suit me. And judgement from others means nothing. Only He matters.