Where can I find a list of which sites in Italy have free admission on Sept 24th and/or Sept 25th? Preferably one in English? Thanks for your advice!
Not sure where to find a list online, but I've been in Italy multiple times for European Heritage Days. Free admission applies to state owned museums, such as the Ufizzi in Florence and the National Museum in Rome. Municipal museums and private museums such as the Vatican Museum usually don't participate. The TI office in the city you'll be visiting those days can provide a list of museums with free admission.
Looking on Beniculturali.it I cannot find current information about this year, but did find things about last year. It is true that it is state museums that are free, and not the Vatican or private ones. It might be worth mentioning, that while they are free, they will be very, very crowded. May be worth visiting on a different day to avoid lines. Just depends on whether you value time or money more! Tough choice in Italy!