Celtic peoples adopted Christianity quickly, easily, and strongly. The conversion of Celtic peoples did not, however, keep them from celebrating some of their old customs. Attempts to replace the year-end custom in the old Celtic calendar of honoring the dead were only partially successful. So Pope Gregory IV put out a Christian alternative to "Halloween" and established "All Saints Day" as the day after Halloween, this was suppose to replace Halloween eventually, but . . . . . . . it didn't.
It is celebrated on Nov. 1 in the West, since Pope Gregory IV ordered its church-wide observance in 837.
I think the reason it didn't catch on was there wasn't any candy given away in honor of the Saints.
Christmas was put in place to replace the Winter Solace celebration of the Pagans. It caught on very well but All Saints day, well it just didn't catch on with the masses to well. If only they had started giving away candy, I'm sure it would have caught on, but those durids started giving away candy and beat the church to the punch.
Keith