Evening is the time when the sky starts to get dark – usually around 6-8 PM.
Night is the time when it is dark and people are generally sleeping.
Say “Good evening” to say “hello” after 6:00 PM. “Good evening” is commonly used at restaurants, in a professional context, and in speeches (when you speak in front of a large number of people):
In general, “good evening” is a little more formal. Friends don’t normally say “good evening” to each other – they would say “Hi” or “What’s up?” or another informal greeting.
Say “Good night” to say “goodbye” – when you are leaving to go home. Family members also say “good night” when they are going to bed.