Halloween evil? No. More on that in a sec, but first we must dig into the nuance surrounding the first question...
Sacrifices? The answer is yes, but voluntary and punitive would seem to be the two ways in which it operated. Their enemies also certainly exaggerated their rituals. So no, my dear dogmatic (watch the movie Dogma) Christians, Halloween isn't based on an evil satanic type of sacrifice where innocent virgins were thrown into a fire!
Hallowe'en, as we celebrate it, was not a Celtic celebration. At the end of the growing season, the festival of Samhuinn - the Feast of the Dead - took place. It not only marked the death of the old season on 31 October, but the arrival of the new year on 1 November. As with other Celtic festivals, it was accompanied by the ritual burning of bonfires and other festivities. The beginning of the new year was also a time to ritually celebrate fertility. It has been recorded that, during the festival, Druids sacrificed animals, such as a male and femal bullock, in fertility rites and burned humans in large wicker cages shaped like animals. It is not known for certain if these human sacrifices occurred, as the veracity of the records is questioned by some historians. The earliest records were made by Romans, during their conquest and occupation of Britain and Gaul. The Romans had a vested interest in discrediting the locals and their practices. Documentation of the forced marches and pitched battles of Roman military campaigns was fairly reliable, but had their moments when venturing outside the military sphere. (Even old Julius succumbed to this, leaving a record of elk hunting in Germania that must have originated from a drunken wag trying to pull the wool over Roman eyes.) As Christianity became established in Italy and spread across Europe, its followers attempted to prevent the celebration of the existing, pagan, festivals.
https://www.answerbag.com/q_view/38755
The best archaeological data supporting Celtic human sacrifice is the body of the man placed in Lindow bog in the first or second century C.E. We actually have the body (well, most of it) so well preserved that scientists were able to analyze his stomach contents to discover his last meal (a partially scorched grain cake). Lindow man was almost certainly a ritual sacrifice; he was strangled, hit on the head, and had his throat cut, in quick order, then surrendered to the bog. This pattern fits the “three-fold” death referred to in medieval Irish tales. What’s more, the man seems to have been of high social rank, and a willing victim...