Very much open to correction on this cos I'm down at work and would have to go home to check my facts but I would assume that it's the mid-point, time-wise, between Autumn Equinox and Winter Solstice, rather than (obviously) October 31st.
Hmm think this one might be open for a toss! I thought it was originally the full moon after the harvest (Sept.) moon, marking the end of the harvest and the onset of winter with a fire festival. The date became fixed when the christian church moved their all-saints day from April I think. But I'm probably wrong again!