The reason it takes a while to get all the Christmas stuff up is because there is sooooo much of it, it's very extensive, and it has to go into 2 parks, the 3 hotels and into Downtown Disney. Halloween Time is not nearly as extensive, and only covers a handful of specific areas of
Disneyland park and that seems to be it, so it does not take as long to get the decorations up or to remove them.
People have reported in other threads (such as the Halloween at DLR Superthread) that there are lots of Halloween in-store and window displays already up. Some of the seasonal treats are probably being sold as well. As for things like the main decorations - the pumpkins on Main Street and everything in New Orleans Square, the Halloween Round-Up, etc., I don't think those will begin to appear until early/mid-next week.
But if all you need are some signs of Halloween Time coming soon, you can find that in the shops and store window displays, and maybe grab some Fall-type treats.
Thanks!
