I came across an interesting post here on WordPress about Halloween, with a lot of annoted info about the ancient Celts, the Druids, Samhain, and the evolution of Halloween festivities. Written from a Christian anti-occultist “ex-witch” perspective, but much more scholarly than most. Among other things, this article points out how little is known, historically, about the religion and customs of the ancient Celts. For example, there’s not enough evidence to know for sure whether the ancient Celts practiced human sacrifice, and it’s not even known for sure whether they had a feast day called Samhain.
However, like a lot of other relatively well-informed writings, this article seems to assume that all Satanists are LaVeyans.