Ah... the debate. I'll post upfront that I no longer celebrate All Hallow's Evening, Christmas, Easter, or artificial Church Calendar Days initially thrown out by Reformed Protestants as unworthy and unBiblical along the lines of the regulative principle of worship vs the normative principle of worship. That is, that God instructs that, “God is Spirit, and those who worship Him must worship in spirit and in truth” (Jn. 4:24). This takes a high view of Scripture as the sole rule of faith and practice, removing especially from church worship any element not explicitly mentioned in the Bible in order to avoid any risk of compromise and obvious violation of what is pleasing to God according to His dictates of appropriate worship. This by-and-large contrasts specifically with Catholic theology or theologies offering God worship based also on sacred tradition instituted by the Church. It harkens back to a time where abuses had crept into the Church based on trying to honour God in man-made fashion. So, overall, this generally translates to the highest view of the Lord's Day over and against most other artificial constructs of a Church Calendar.
Now, having said that, and having acknowledged I don't religiously observe Hallowe'en or many other such days, I can also somewhat defend those Christians who do, or who especially are intent to honour God in every aspect of their lives including holiday observances. Yes, Hallowe'en has pagan origins, and yes, Christians eventually attempted to co-op most pagan cyclical observances into the Church Calendar as a missionary tool; but All Saints' Day, honouring the sacrifices and martyrdoms especially pertinent to early struggles of Christianity in very hostile environs is rooted in the first high honour of worship allocated by the Church outside Lord's Day observance. That is, placing a premium of observance and recognition of a saint's homecoming into heaven was of special importance long before official designation of Christmas, Easter, or other days now so popular among Christian observances. These are the Christian roots of Hallowe'en, as a Christian Memorial Day.
Further, abuses surrounding saints' relics, certificates of indulgences, and the like, was the prime reason Hallowe'en was chosen by Martin Luther to launch a call for Church reforms that would become the Protestant Reformation. Some today even replace a Hallowe'en observance with a Reformation Day observance, since they exactly coincide. So, whether from putting a focus on spiritual reformation or on a memorial honouring of those Christians gone before us to their eternal reward, there are reasons of celebrating that would seem honourable. And as others have pointed out, one other take of redeeming Hallowe'en away from pagan ritual is using the day or evening to focus on the very real terrors of sin and Hell awaiting the unrepentant – a sort of, “Sinners In the Hands of an Angry God” approach.
I still say the greater part of wisdom and holy comportment is to avoid any religious observance of the day, but there are uses of it that can be put to hopeful well-intent.
