First of all, Paul sides with those who want to eat the meat sacrificed to idols because we, in fact, do have the freedom to do so. Eating meat sacrificed to idols will not, in and of itself, get you thrown into hell, much like participation in Halloween, in and of itself, will not jepordize your salvation. However, if you would have read the rest of my post, which it seems you did not, you would have seen that Paul then suggest that, even though we have the freedom to do these things, actually doing them might not be the best of ideas.
I guess I should have laid some more groundwork to make my position a little clearer.
Now, I myself would not consider Halloween a "harmless evening of fun". While you might think that there is no sacrificing or paganism to Halloween, I think it is extremely obvious that there are some basic, entry-level associations with the demonic and the occult that are intrinsic to this day. And, as the influence of the occult increases in modern culture, through what some people may consider harmless activities, such as participation in Halloween, ouija boards, or even through television shows like “Buffy the Vampire Slayer” which actually glamorise the demonic and the occult, so increases the lure of the demonic and the occult, for those who don’t believe in God as well as for those who do.
In regards to Halloween, if we participate in something that might cause a weaker brother or sister to stumble, let alone give the impression to the non-believer that Halloween is, indeed, a “harmless” holiday, then maybe this, more than anything, is the best argument to put aside our freedom and avoid this holiday. Again I say, edification is more important than our personal gratification.
Listen, if you want a verse that says "Do not do this thing" then you won't get it. It doesn't exist. If, however, you want to set your heart and mind towards something a little closer to "Thy will be done" rather than "My will be done", Scripture suggests that you should avoid any customs, such as the practice of Halloween, that glorifies that which God calls evil. This might mean avoiding anything that glorifies the occult, witchcraft, the devil or the demonic, death or violence or promotes these things as positive and/or fun (i.e. costumes, decoration, music and stories that glorifiy the demonic and the occult).
Another persons edification is more important that your personal gratification.