With an update on their Developer website, Apple confirmed the annual iTunes Connect shutdown for the holidays that will prevent developers from releasing new apps and updates:

The busiest season on the App Store is almost here. Make sure your apps are up-to-date and ready for the winter holidays. New apps and app updates will not be accepted December 22-29, so any releases should be submitted, approved, and scheduled in advance. Other iTunes Connect functionality will remain available.

Because of your incredible apps, the App Store crossed 100 billion cumulative downloads. Revenue from the App Store increased 25% year over year, and the number of transacting customers grew 18%, setting a new all-time record. We want to thank you for all your hard work and dedication to our platforms.

As usual, developers who are making a new app or preparing an update should plan accordingly.