Technically, little bits and pieces of holiday decor begin to appear right after the Halloween decorations come down - so basically at the beginning of November you will see some things. You will begin to see some garland going up here, some ornaments and different-sized trees there; some lights and holiday food items become visible.
However, if it is possible for you to arrange getting to DLR during the 'official' holiday season, which begins this year on November 20, I think, I highly recommend you do that (rather than going in early November). That is when the Castle/Main street light shows and the 'snow' are in full swing, IASW Holiday is stunningly, jaw droppingly beautiful and everything is very Christmasy. It is worth waiting the extra few weeks between early November and November 20th just to see DLR in its full holiday splendor.