Ciao Joao,
We update this article every year with info on what's open in Florence on the Christmas holidays - so take a look since it will likely be very similar, with some updates done a few weeks before.
http://www.visitflorence.com/christmas-holiday-museum-hours.html
Sounds like a great plan! generally all churches will be open for mass/services but not for visits so it is a good day to enjoy all of the outdoor architecture, the monuments and piazzas that are all in themselves things to see and admire![]()