[hatthew]

We discourage changes starting early/mid november and completely freeze deploys a week before thanksgiving and christmas, but there's still so much that can go wrong. The other commenter summarized it well, but in addition there's also systems that require continuous automatic updates as part of their functionality, and sometimes those automatic updates can start to go haywire. For some systems, the holidays are the most important part of the year, and it's extra crucial for people to be monitoring and ready to respond at a moment's notice.

[erik_seaberg]

There are also systems that aren’t entirely stable due to resource leaks, and when you stop deploying you might find you need to restart them occasionally.