That's a very good question someone is bound to ask from time to time. Here is an answer:
Well, a while back some Ugandan church leaders came to visit
a friend of mine. They came to visit
right near the time of Halloween. And
one of the Ugandans, as he was visiting in and around town, asked my friend Tony in a rather somber voice, "Tony, you don't celebrate Halloween, do you?"
And my friend Tony replied, "No. No, I don't."
The Ugandan was keenly aware that there is an unseen,
yet very real, spiritual world. And even
though it may be lighthearted, it is not a good idea to be dabbling in things
pertaining to witchcraft, the demonic, or vampires and such things.
So, for that reason, my wife and I don't like to support Halloween
either, even if we were only dressing our kids in ninja or princess
costumes. Now, having said that, we do have a hard time passing up an occasion where entire neighborhoods of strangers freely
come to your house and/or freely welcome you to theirs. With that in mind, if
the occasion can be used in a harmless* way for the proclamation of the Good
News, then we tend to be more amenable to it. If it's possible to dress up as a ninja and pass out a biography on someone who experienced God mightily... then hand me my mask and plastic saber.
* - Harmless
meaning "not wearing costumes trivializing the reality of satanic power"