[ And cue the very animated story-telling. ] People believed that at this time of year, before All Hallows' Day, the wall that separates the realm of the living and the dead is weakest! And so spirits of the dead roam freely among the living.
But! They can get lonely and it's said that they'd take the living back with them into the realm of the dead! So people wore masks and dressed up like the dead so they won't be taken!