I just got this in one of my Rome Newsletters. FYI
European Heritage Days – September 24-25 Visitors in Rome September 24-25 can enjoy free admission to the state museums and archaeological sites. This includes the Colosseum, Roman Forum, Palatine Hill, Capitoline Museum, National Museums of Rome, among others. The goal of European Heritage Days is to encourage people to learn more about artistic and culturally significant sites.
It's not just Rome. State-owned museums and sights throughout Italy are free on September 24-25. Private museums like the Vatican Museum don't participate. TI offices can provide a list of participating free museums. France celebrates European Heritage Days the weekend before, September 17-18. Same deal - Lourve, d'Orsay, etc are free.
Glad to hear this, will be in Venice this weekend!
Keep in mind, that since entrance is free, the museums are VERY crowded. Depends on if you value your time or money more!