Married to a old Roman Catholic (a hippie), I have heard a great deal over the years about the struggle between the old pagan religions and holidays of the early Christian church in Europe. Along with some reading, my interpretation is that in trying to bring non-believers into the realm of the Holy Catholic Church, there was an "if-you-can-beat-'em-join-'em" attitude, where Catholic holidays were celebrated on pagan holidays. Over time, the pagan significance of the day dwindled and was mostly forgotten, while the overtones of the Catholic holiday were remembered (at least by a majority of people).
The Research Center of The Library of Congress notes:
The Celts believed that at the time of Samhain, more so than any other time of the year, the ghosts of the dead were able to mingle with the living, because at Samhain the souls of those who had died during the year traveled into the otherworld.
The site goes on to note the change of the face of Samhain with the arrival of missionaries:
Samhain became the Halloween we are familiar with when Christian missionaries attempted to change the religious practices of the Celtic people.
I've listed the URL of the website below.
I love this kind of history...