Mackysmycat --
Do you mean, when do the decorations begin appearing around the parks?
Or do you mean, when does the holiday season at DLR officially begin, with holiday entertainment in full swing, etc.?
I have to ask because they are not one and the same.
If you just want to see decorations going up (and some folks are fine with just a bit of Christmas), then, as figment_jii said, some things will begin trickling out in early November. Actually, some decorations and merchandise items will probably appear before Halloween.
I don't know what the status of the Christmas parade and holiday fireworks will be this year due to the 60th anniversary events, but if they return they could possibly soft open over the weekend after Halloween weekend (in other words, over 11/6-11/8), before the holidays have officially begun.
If you want to enjoy the holiday season after it has officially begun and seasonal entertainment has kicked in, you'd probably have to wait until at least 11/12. The season will likely officially begin somewhere between 11/12 and 11/16.
If you want to be at DLR when the hotels are decorated, the hotels' Christmas trees will probably begin to go up around 11/17 or 11/18 or so. Santa Claus, the carolers and some extra decorations won't appear at the hotels until Thanksgiving and beyond.
As figment_jii mentioned, stay tuned to the Christmas/Holiday Season Superthread. We discuss the seasonal developments there all year, including whenever anything leaks out or gets announced, and I have a post on page 1 that will run down when everything starts as we learn more.