First, like many another holiday, the retail industry and general secular culture have succeeded in secularizing All Hallows' Eve. It's a time when people can gratify their desire to dress up and role-play, and kids can get candy and have fun.
Second, it derives more than a little of its imagery from the Pagan holday of Samhain, one of the "cross-quarter" feasts of pre-Christian Celtic Europe. (A "cross-quarter day" is halfway through a given season. Samhain/Hallowe'en iis approximately halfway through calendar Autumn.)
All Saints' Day is the date on which the good Christians ("saints" in the Pauline usage) who have gone on to Glory are remembered, as opposed to the days on which specific people whose lives struck a chord in people throughout the church went to be with Jesus. October 4, for example, is the date on which Francis of Assisi died and his soul went to heaven; December 6, Nicholas of Myra; February 14, Valentinus. But while your grandmother and mine may have been every bit as committed a Christian and lived a life as deserving of commemoration as these folks, they haven't been recognized by the Church across continents and decades as having their own saint's days. November 1 is the day when historically the Church as a whole remembers, collectively, these folks who were "saints" in the sense Paul uses the term. And like all Christian feasts, it begins at sundown the day before -- October 31.
In doing this, the Church knowingly co-opted and Christianized the holiday of the pre-Christian Pagans, just as, three days following the winter solstice falls the day on which daylight is noticeably longer for the first time, after six months of growing shorter, and the Romans declared this the Birthday of the Unconquered Sun. And, though the pun only works in English, we Christians took it over as the Birthday of the Unconquered Son.
