afternoon, evening and night

Afternoon changes to evening when it starts getting dark, more or less.
However, it depends on the time of year.
In summer, we stop saying afternoon by six o'clock, even if it is still light.
In winter we go on saying afternoon until at least five o'clock, even if it is dark.

Evening changes to night more or less at bedtime.

Note that Good evening usually means 'Hello', and Good night means 'Goodbye'
— it is never used to greet people.

• A Good evening Terrible weather, isn't it?
  B-Yes, dreadful.

• A:Hasn't stopped raining for weeks. Well. I must be going. Good night
  B Good night