@kheredia, there are some religious people who won’t countenance a holiday that isn’t found in the Bible. (Oddly enough, those people usually do drive cars, use telepones and washing machines, etc.) But the ecclesiastical calendar of the Catholic church from early times has been full of feasts of all kinds, and many of these celebrations have their roots in the Middle Ages, when the Church was the most powerful institution in the Western world.
All Saints Day and All Souls Day, November 1 and 2, are two such, and the old idea was that on the eve (the night before) of those two big holy days (holidays), which together cover just about everybody—all hallows—the evil spirits might run loose while they still could. So certain customs grew up to protect ourselves from the dangerous night when they were all abroad (looking like goblins that might come to our door). We might make offerings to them (trick or treat!) so they would leave us alone.
Meanwhile, it was also a big night for the practicing witches! So there really might have been dark celebrations going on while the good Christians were quaking behind their doors and waiting for the holy day to dawn.
The kids in costumes going from door to door for candy treats have no idea what they’re acting out, but it is a day that has deep roots in the culture from which our culture grew.
I happen to believe that Hallowe’en has become such a big deal in the U.S. in the past few decades because it is perceived as a secular holiday (by people who don’t know a thing about the history)—and therefore that it is safe and politically correct to play up in schools, just like Valentine’s Day and others that seem to be free of religious connotations. A great sensitivity has arisen about giving official school attention to a traditional and legal U.S. holiday like Christmas, which is really odd when they’ll give over an entire day to celebrating a minor Mexican holiday as an official school event, but anyway. So the emphasis on Hallowe’en and Valentine’s Day has taken the place of the old emphasis on Christmas and Easter. And so merchandisers follow through, and they become a big deal at Walgreen’s and Safeway, and presto! everybody thinks it’s a major American event.