In the MK, the Christmas Parade starts 11/10. Epcot and HS don't start their celebrations until 11/28 so I don't know if they are decorated before that. AK and DTD haven't released dates yet for their celebrations. At the Disney World website, click on Calendars on the home page and than Special Events. It will bring you to all the Christmas celebrations.
We went last year, the week after Thanksgiving and all the parks were fully decorated. It is very spectacular and done Disney grand. Do not miss the Festival of Lights at HS or the Christmas Parade at MK. In both places they make it snow.