The Pagan/Wiccan festival of Samhain comes from an old celtic word, Samhain (pronounced sa-ween) literally means "summers end"
It's the Pagan "new Year". Wiccan's see the year as a "wheel" which is constantly turning, Samhain is at the end of that wheel, and so the next Sabbat, Yule, is seen as part of "next year's" celebrations. This all dates back from before the time when man had invented the concept of the calendar. Samhain is recognised by Wiccans as an intercalary day - a day when the veil between our world and the world beyond is at its thinnest, and is therefore a great day for divination - tarot readings, seances, scrying etc.
I think that where Christians (or anyone else from any other religion for that matter) have a problem with Samhain is that it's a night where the spirits of those who have passed on are supposedly most likely to be summoned, and in the burning times witches were alleged to be in league with the devil, so samhain was the most "evil" day imaginable.