Rome on Christmas

The Pantheon will be open from 9a-1p because it is now a Catholic Church, as will other churches. I would avoid St Peter's unless you are able to get a ticket for mass with the Pope. The squares and piazzas are always open. There is an exhibit of Nativity scenes that will be open on Christmas day, details http://www.rometravelcafe.com/events/details.asp?EventID=577

The museums, forum and other archeological sites will be closed. But there will be enough restaurants open, so you won't go hungry and many of the churches will be open, even between services so you can visit them. There is also a Christmas market in Piazza Navonna. We went last year and just had a wonderful time walking around Rome and people watching. Many Romans put on their best clothes (furs, jewelry, high heels, etc.) and walk around town. It was a delightful Christmas!