When used as a farewell, these two are basically interchangeable. However, "good evening" can also be used as a greeting. "Good night" is never used as a greeting. If you walked up to someone at 3:00 am and said "good night", they would be confused. You could say "good evening," even though it is not really "evening". If you leave at 3:00 am, you could say either "good evening" or "good night", but "good night" would be much more common.

I think it's best to remember that GOOD NIGHT is like GOODBYE. Never say it when you meet someone, say it when you go. GOOD EVENING is more flexible. You can use it when you meet, you can use it when you go. If you are using it instead of (or with) goodbye then you have to decide when it is late enough to change good EVENING into good NIGHT. Only you can make that decision. Actually, for me, if it is dark I habitually use GOOD NIGHT when I go, or they go. For me, good evening is the greeting, and good night is the goodbye. If it is dark! If it was a really light summer evening, and 10pm (in Scotland it can be light at 11pm!), I probably would just say goodbye... then I wouldn't have to work out if it should be good evening or goodnight! Other people habitually say "Good evening to you," as they go, even if it quite late. There is no law to say you have to use one form or the other. It's personal choice and style. Just don't say good night when you MEET! And if confused on leaving, just say goodbye!

I use "Good night" for just before going to sleep. Sometimes, I also use it instead of "goodbye" after say 9.30pm e.g.at the end of a session with a student after this time.

It is a polite phrase used whenever you a) are leaving someone, and b) it is nighttime. It CAN be used just before going to sleep, and often is, but it is not restricted to that. The two things that popped into my mind, oddly enough, were the once-famous closing lines of two famous television news programs. In both cases, the programs ended well before bedtime. The Huntley-Brinkley Report, with co-anchors Chet Huntley and David Brinkely always closed: ""Good night, Chet. Good night, David. And good night, for NBC News." While Edward R. Murrow always closed with "Good night, and good luck."

'Good night' means 'Goodbye'. You say it when you, or the other person, is LEAVING, and you are not going to see them again before morning. 'Good night' isn't just used when you are about to go to bed, though. If you are leaving your place of work in the evening, especially this time of year when it is dark, you could even say 'Good night'. 'Good night' is NEVER used as a greeting when you meet someone. When you meet someone, you say 'Good evening', however late it is. Or in other words, after dark you say either: Hello = Good evening Goodbye = Good night.