Samhain was the most important festival to the Celts, and they believed that at this time of year, dead souls re-entered the world and mingled amongst the living—and that fairies, ghosts, and demons pranced about as well. This rather freaked out the Celts—as it would any of us. Consequently, they build large bonfires on Samhain, both to celebrate the festival and to keep those other-world spirits at bay.
In 601 CE, Pope Gregory the First—aka “Gregory the Great”—issued an edict to his missionaries that basically said this (and please do forgive my paraphrasing): "Those Celtic cats sure do strange things and have strange beliefs! If only we were dealing with rational people, with people prone to sensible beliefs like ours! (E.g., the Immaculate Conception; Eve arising from Adams rib; and the world going to hell in a handbasket because of an apple.) Because the natives are set in their ways, I think forcing them to change will be a major headache. So I tell you what…let’s go ahead and allow them to celebrate their wacky festivals, but impress upon them that good will befall them if they celebrate the festivals in the name of Christianity, and evil will befall them if they continue celebrating in the name of their crazy gods and beliefs. That should scare them into submission! We’ll call this Samhain thing, where they celebrate the coming of winter and all things dead, “All Saints Day.” Corresponds pretty well, right? Oh, and that mid-winter thing they celebrate, when they cut down trees, let’s tell them that’s when Christ was born—say, like on the 25th. That should work. Later! Be careful out there!”
(Note to readers: As I said, that's not the pope's exact wording, but I'm pretty sure the wording was very similar to this….)