Answer:
In Italian "Buona notte" means "Good night." However, in Italian "Buona sera" is used as a greeting in the evening and night hours. It is the equivalent to "buongiorno" in the day. You only say "Buona notte" if you are saying "Good night" in the context of "goodbye." Never use "buona notte" as a greeting. For example, if you meet some friends at 11PM you say, "Buona sera!" But if you have just spent an evening together and you are saying goodbye to your friends at 11PM you say, "Buona notte!"