I believe Christmas stuff starts going up right after Halloween, and is usually finished by mid- to late November
Current hat total: 100 of 100 - completed!
Duckland: Stroll through downtown Duckburg, making sure to tour Scrooge's Money Bin, then cross the Audubon Bay Bridge to St. Canard. Visit Liquidator's Splash Pool, and don't forget a stop by Bushroot Gardens!
Decorations start going up slowly around the park very soon after Halloween. It really kicks in around the middle of November and usually everything is up and on by the weekend of Thanksgiving.
Small World is looking to come up around the end of November, as far as I understand, and if the facade is finished I would hope they'd light it up with the rest of the park, though I can see guests assuming that the lights and decorations mean the attraction is running.