From beliefnet, by a Catholic priest who is an assistant professor at the University of Virginia. If its origins aren't pagan, where did it come from?
"It’s true that the ancient Celts of Ireland and Britain celebrated a minor festival on October 31 - as they did on the last day of most other months of the year. However, Halloween falls on the last day of October because the Feast of All Saints, or 'All Hallows,' falls on November 1. The feast in honor of all the saints in heaven used to be celebrated on May 13, but Pope Gregory III (d. 741) moved it to November 1, the dedication day of All Saints Chapel in St. Peter’s at Rome. Later, in the 840s, Pope Gregory IV commanded that All Saints be observed everywhere. And so the holy day spread to Ireland.In my youth, long ago, our church held an annual Halloween party. There was a costume competition, bobbing for apples, and a good time. More often than not the costume competition was won by a senior saint. Obviously, it had nothing to do with Satanism, worship of Mother Earth, primitive feminism or any other "ism." It wasn't about remembering those in Hell or "Purgatory" either. It was a lot of fun.
The day before was the feast’s evening vigil, 'All Hallows Even,' or 'Hallowe’en.' In those days Halloween didn’t have any special significance for Christians or for long-dead Celtic pagans...
...What about those in the other place? It seems Irish Catholic peasants wondered about the unfortunate souls in hell. After all, if the souls in hell are left out when we celebrate those in heaven and purgatory, they might be unhappy enough to cause trouble. So it became customary to bang pots and pans on All Hallows Even to let the damned know they were not forgotten. Thus, in Ireland at least, all the dead came to be remembered - even if the clergy were not terribly sympathetic to Halloween and never allowed All Damned Day into the church calendar." [The article, with more of the history.]