During that time, iTunes Connect will not be accessible and you will not be able to submit any apps or In-App Purchases. Processing of any Newsstand Atom feeds will be delayed until after December 27. Sales and Trends reports via the Autoingestion tool will be available but may also be delayed.
Also, you should not schedule any pricing changes or apps to go live between those dates. Pricing changes scheduled to take effect in that date range will cause the app to become unavailable for purchase until after December 27. Similarly, apps scheduled to go live during that range will not be released until after December 27.