"When I was a kid in Lancashire in the 50s, there was no trick or treating - but there was a custom known locally as "mischief night", (Will Fly)
As I recall mischief night was "celebrated" on 4th November in my part of Cheshire. I don't recall any Halloween (31st Oct.) activity there at all when I was young although, of course, there were All Saints' and All Souls' days.
"It implies that there a secure barrier between the (mainly malignant) world of the dead and our own one, something we can count on for 364 days of the year." (Jack Campin)
According to Wimberly there is an Irish text for Tam Lin which puts the fairy processions as taking place on the 1st May. There are also plenty of folk customs based around May-eve which are based around contacting "spirits". There are also plenty of other ballads that have visitations, albeit mostly revenant ones, taking place on most nights of the week!
"Celtic religion went out in Britain when the Romans arrived. Roman religion took its place - including the worship of dead emperors. Then that was replaced by Christianity. ….. The chances of anything surviving from the Celts are minuscule, and the evidence that anything has survived is slim to none." (Phil Edwards)
"Christianity likewise enshrined a lot of pre-Christian ideas / festivals / concepts that remain with us into a new era of secularism." (Jack Blandiver)
I'm inclined to agree with Jack Blandiver here. I'm not sure that it's all that easy to remove people's underlying beliefs and the success of occupying forces in imposing a religion are questionable. So the arrival of the Romans, and the fact that they left evidence that they were still worshipping their own gods, does not mean that the rest of the population, away from the forts, followed suit. Christianity, being partly home-grown, is another story but we have plenty of evidence that the Christians, at least at the start, were more inclined to absorb many of the earlier practices and celebrations into their own than to banish them. (That seems to have been more of a 13th – 17th Century phenomenon). It's worth looking at Pope Gregory's letter to Mellitus (around 605) for contemporary evidence of this. Additional possible evidence for the survival of non-mainstream practices over time can be found in the Pellar tradition of West Cornwall, which, though I'm inclined to think that current "practitioners" are more likely to be influenced by Gardener and Crowley, was documented well into the 20th Century. There are also the interesting "witch pits" found in Saveock Water in Cornwall which seem to date from 17th Century to the 20th.
None of this, of course, has anything to do with "Celts" (a recent catch-all term useful as a sales pitch) but I think it would be wrong to assume that practices (and beliefs) that we now class as folklore cannot have an earlier origin than recent Christianity. To paraphrase Jack Blandiver, the metaphor "thinning of the veil" may be a recent one but the underlying idea, and the evidence for it, may date back a lot further.
(Sorry, I seem to be agreeing with Mr B a lot - a lapse I'll try to address as soon as I can.)