The only museum that is open on December 25th is Palazzo Strozzi where you will find the Jeff Koons exhibition at the moment. I believe Palazzo Vecchio might also be open, but I'm currently checking all the museums out to see their hours for the holidays and don't have a full idea of whether Palazzo Vecchio has confirmed for this year. I'll post an update as soon as I'm done, if you can wait another day or two to finalize plans. If you're eating out, make sure to call and book your restaurant to ensure you have the table.