We went at the end of August/beginning of September and got to experience the park being normal one day and literally the next day seeing everything decked out for Halloween. It was awesome. I think it is smart of Disney to give the holidays at least a 2 month time frame. It gives guests who may not be able to see the decorations a chance to experience them.
And Halloween and Christmas are the only two that get such attention. I used to go in February and would see some Valentine things but the minute the 14th was over, it got immediately switched to St. Patrick's day stuff. So the transitions are very fast. And you only know it's Easter when you see the Easter Bunny and the southern belle ladies on Main street Easter Sunday. I feel sorry for travelers who don't get to see them if they come in too early or late. At least with Halloween and Christmas, vacationers get some leeway to experience everything.
As other posters have said, Disney makes everything known and accessible to travelers; the information is out there. If you are uncomfortable walking on a street with a pumpkin on it, don't travel in September/October. There are 10 other months to travel in. Me? after seeing how totally awesome the place looked, I am now planning a real Halloween trip for next year because I want to go to the party and all that stuff...so yea...Disney's advertising worked for me, lol