The common story goes that Halloween originated in the misty days of pre-Roman Ireland, with the year-ending festival of Samhain. That final day of the Celtic calendar was a "thin time" when spirits walked the Earth and costumed junior Druids traveled from home to home with lighted turnips, begging for food. The festival was appropriated by the Catholic Church as All Hallow's Eve as a fair or foul attempt to convert the Pagans, and evolved over time into the holiday we know today.
If only there was any historical evidence for this story!
Very little is actually known about Druids, their festivals, and their practices, on account of their being a pre-literate culture. Most of what we do know comes from the Romans, an imperial force who cannot be relied upon to have a full, nuanced appreciation for the cultures they attempted to conquer. It was the Romans who gave the impression that mass human sacrifice in Wicker Men was a common Druidic practice. After the collapse of the Roman Empire in the West, Celts took to Christianity and took to it hard. So it can safely be said that there was a festival surrounding Samhain, a term which literally means "summer's end" but was not necessarily the end of the Celtic year. It may have had something to do with honouring the dead, but we don't know for sure, and that practice may have been Christianized as All Saints Day, a lesser festival honouring all the saints and martyrs who did not have their own designated feast days (the preceding evening being All Hallows Eve), and followed by All Souls Day remembering all the Christian dead. Yet the original practice of All Saints Day varied from country to country - November 1 in England and Germany, April 20 in Ireland, May 13 in most of the Christian world - and the November 1 date was only fixed in the 12th century, well after the Christianization of the Celts. Scholars can't actually say what transpired during Samhain festivals, on account of there being no record whatsoever. It seems that processions for the faithful dead were actually a Christian invention, as well as the door-to-door begging for food. All Saints was only one such opportunity for such activity: processions and door-to-door hunger appeals also surfaced on the feast days of St. Andrew, St. Nicholas, St. Thomas, and even later on Guy Fawkes Day. Like other holy days, it became an opportunity for ribald fools festivals, danse macabre, and pranking. In Europe and the British Isles, Halloween is only a minor practice, oftentimes unwelcome, and one that has mostly been imported from the United States.
That being the case, where did the idea that Halloween was an ancient, pre-Christian Druidic practice come from? The most likely answer is that it came from the same people who invented the modern holiday of Halloween itself: the Victorians!