Well, not necessarily, because All Saints' Day was moved to November 1 by the Pope of Rome (it was originally in the late spring, and still is, in the East), and I don't think "ancient Celtic observances" had much to do with it.
Pagans celebrated stuff around that time because it was the end of the harvest in N. Europe, just like the celebrated stuff in the spring (for obvious reasons) and around the winter solstice (yes, they did astronomy, too). It doesn't follow that anything celebrated around that time is pagan.
The Catholic Church had the quite genius (IMHO) practice of making it easier for the pagans to convert by coopting or allowing parts of their rituals and putting Christian events around the same dates. This is how you get the Halloween dress-up, Easter Bunny, yule logs, Christmas trees, etc. for rituals and Halloween, Christmas and Easter for dates.