On the Radio 4 current affairs programme PM this afternoon, a folklorist was asked about the origin of Hallowe'en. He suggested that there were three alternative origin stories: the Christian festival of All Hallows' Eve, the Roman festival of Pomona (the apple goddess) and mumble mumble Celtic, which people were doing two thousand years ago. Of the three, the folklorist said he liked the sound of the Celtic mumblety thing the best.

I thought all this was pretty confused - after all, Hallowe'en doesn't come from All Hallows' Eve, it is All Hallows' Eve; whether celebrations of All Hallows' Eve borrow from pre-Christian seasonal rituals is a separate question. As for whatever the Celts were doing 2000 years ago, to me this raises a number of questions: (a) do we know what they were doing? (b) no, seriously, do we actually know? (c) to the extent that we know anything, are we sure we're not cherry-picking (mistletoe-picking?) - that is, homing in on odd bits of lore that appeal to us precisely because they resemble our own traditions? and (d) how are these 2000-year-old rituals supposed to have survived no fewer than three changes of religion as well as the passage of, well, 2000 years?

But my wifty-wafty-o-meter really pinged when Folklore Guy said that traditionally (in Celtic mumble country) this was the time of year when the spirits walked abroad, and more specifically when the veil between this world and the next was at its thinnest. It's a very familiar image - which, given that I don't read much Folklore Studies, made me wonder.

Folklore people: where does this idea of a veil between the realms, which gets thinner at certain times of year, come from? Did a particular collector or folklorist bring it into circulation, and if so who?