Some people use the calendar to denote the day when fall starts, as for me I consider it fall when friends from high school send me the latest "proof" that Halloween is the devil's holiday. While I can't find the e-mail from last year, I asked Fr. Matusiak from OCA.org, and he said there was nothing wrong with it. Obviously you can't take part in pagan worship nor should you play with Ouija boards. However, there's nothing wrong with trick or treating, bobbing for apples, and the vast majority of other Halloween traditions.
Honestly, I think this is one of the most hypocritical aspects of the fundie/evang protestant world. Christmas is, of course, not a question for all by the extremist types. However, it (just like Halloween) is a replacement for a pagan holiday (Yule). No one choses to focus on the pagan origins of Christmas trees, Yule logs, and Mistletoe. Yet for some reason the double standard for Halloween, in which everything is measured by it's "pagan" origins. Halloween customs of today are as much, if not more so, divorced from their pagan origins as any Christmas tradition. It's just a remaining Puritian influence, of course they hated Christmas as well for being both a celebration and a Catholic holiday. If it wasn't for the "satanic scare" of the 80's we wouldn't have all this left over anti-Halloween rhetoric. The fundie/evang protestant America become absolutely convinced (and obessed) of the idea thousands of practicing satanists were living among them killing goats and virgins to honor their dark lord. No evidence of this was actually produced, and yet here we are some 20 years later with the "devil's birthday" crowd still going on and on about it.