This is something I had previously taken for granted as an established fact about Hallowe'en: that it was on the same date as a Celtic pagan festival called Samhain and that a lot of our modern Hallowe'en traditions (like jack o' lanterns, costumes, trick or treating etc.) were inherited from this older holiday. This seemed to make perfect sense until I found out that most of these traditions are either modern or medieval in origin. Which made me wonder, what (if anything) do we actually know about Samhain and does it have any connection at all to the modern holiday of Hallowe'en?
The Celts are a very broad term as you had Celtic peoples from Ireland to Turkey. But the Celts that we're talking about are the Gaels who populated Ireland, Man, and Scotland and whose descendants live there today.
According to historian Ronald Hutton in "The Stations of the Sun", evidence is scant on Samhain in pre-Christian times. We know from early Christian monks writing about the Irish that their folklore viewed Samhain to be significant. But that's about all we know.
It's likely that a great deal of the association with the dead and spooky things originated from the Christian festivals of All Saints Day and All Souls Day.
Despite modern Pagans believing otherwise, the historical record doesn't indicate that it was a Celtic New Year.
There just isn't enough information but as Hutton's work shows, it rarely validates the beliefs of modern NeoPagans.