A little more than a year ago, ArchAngel said in this thread that the celebration of Halloween is a bit like what Paul wrote about eating food that had been sacrificed to idols. I think that's a great illustration. There were people who used to avoid eating meat that had been sacrificed to idols and then served at dinners, for moral reasons. There were other people who had no problem at all with repurposing that meat for their own uses. They viewed all meat as good, nutritious food, and placed no value on the fact that it had been used as a symbol of something pagan.
There are people in modern times who avoid celebrating Halloween, and even Christmas and Easter, because of moral objections about the holidays' origins. Then again, there are other people who have no problem repurposing these days for their own use in worshipping God, entertainment, or whatever. They view these days as being fine days for special events, just the same as any other day in the year, not tainted. They place no value on these days' pagan origins or the symbols that had been used for pagan worship.
In both the modern situation and the ancient situation, people can go either way. They can say the pagan deities are false and have no meaning, or they can take steps to avoid even the appearance of evil. Take Paul's advice to heart, whichever side of the issue you're on. Do what you feel comfortable with. If you feel that celebrating is innocent, then it's fine to celebrate. If you view celebrating these holidays as an act of worship to God, such as Christmas or Easter, then certainly worship the Lord on that day. If you feel that participation would be an act of honoring demons and false gods, then don't do it no matter who invites you to a party. If you abstain, don't look down on people who participate. If you participate, don't look down on people who abstain, and don't push them into situations where they may feel that they are dishonoring God. If you want to take it a step further, like Paul wrote, you might want to consider not even celebrating a perfectly fine holiday, if you think your celebration creates a temptation that could lead someone else to do what they feel is equivalent to worshipping false gods.