The important thing to understand saying about 'Good night' or just 'Night' to someone is that is means 'Goodbye'. You ONLY use 'Good night' when you are taking your leave of someone, and it is the last time you are going to see that person that day. By contrast, 'Good evening' means 'Hello'. For example, when you enter a restaurant for dinner, the staff will greet you with a 'Good evening', and then say 'Good night' when you depart - the actual time on the clock is immaterial. 'Good night' usually said during the hours of darkness, but otherwise it doesn't matter what time it is. For example, especially in winter, you could say 'Goodnight. See you tomorrow' to your colleagues at 6 pm. It doesn't mean that you are going to bed - it just means that you are parting for the last time that day. Whatever anyone told you about midnight being a cut-off point is nonsense - ignore them. Just remember that, once the working day is over, 'Good evening' means 'Hello' and 'Good night' means 'Bye'. That's all you need to know. Let me know if that makes sense, Emarbe. Would be nice to hear from you again!

"Good evening" is a greeting. You say it when you arrive someplace. "Good night" is another way of saying "Goodbye." You say it when you are leaving.

Night could be said to begin at sunset, after the sun goes down. At night it is dark outside. Good night can be said at any point during the night.