I keep checking and keep being disappointed by the 9a opening. We’ll be at WDW 12/16-12/23 and I want to plan, plan, plan! I expect a change of park hours and the addition of some EMH, but WHEN can I expect them? I have 8am ADRs to cancel!!! 
There is an update in hours around the 15th of the month for the following month. It usually starts updating on a Friday and finishes up Monday.
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As @Incredible_Family Mentioned, hours are updated on the 15th of the month prior, but I’ve also been told that with Christmas, they can, and often do, update the hours even during the week of.
Absolutely they do!
Last year the Christmas to NY week hours were updated just a few days before we flew out, so around the 20th I think. That was to add EMH to MK every day from 7-8am, also late nights were extended too. And some of the other parks had regular opening hours altered then as well.
MK hours on Xmas party days have been expanded to 8a-6p so I released my 8a BOG ADR.