I meant they would only be open for services as smaller churches can't be manned all the time. Churches in big towns or cities will be open all day, but they will have a sign stating what times services are on and ask any visitors to respect that and not to visit or to take photos. You can go in, but wandering around to look would not be the right thing to do.
Oh, thanks for the clarification. Actually, if it is Catholic Church, I would prefer to go to Mass, when visiting a church. But, this would probably limit the number of churches I could visit to Saturday evening and Sunday morning. Unless, of course, in Italy Mass is held on a daily basis. Churches in the U.S. use to be like that, but not so much anymore, that I am aware of, at least.