One of the best things to do to find out if something you ewant to do is open or closed on a specific day, whether holiday or other, is to simply look up their website or Google "XXX museum hours." Even if the site is in Italian you can probalby decipher if it will be open or not. Typically it will say something like Giorni di chiusura Lunedì, 1 gennaio, 1 maggio, 25 dicembre meaning Days closed Mondays, January 1, May 1 and December 25. It's a litle tedious to have to do this for several sites but it's useful. BTW, the Colosseo, Foro Romano and Palatino are open daily. I know they are open Dec 26 and I think maybe even Christmas Day. Too big to close? Also, just walking around to sites that are not covered by admissions and passes can make a lovely day. The Appian Way, the Gianicolo, the Aventino, the many piazzas, fountains and ruins. Or go up to Tivoli to Villa d'Este, days closed Mondays, January 1, May 1 and December 25. Always crowded on Italian holidays that they are open, though.