Buenos días
Used the same way Good morning is used in English - to greet someone any time before noon.
Buenas tardes
Generally used between noon and 6 PM. The Spanish equivalent to Good afternoon.
Buenas noches
Literally translates to Good night, but can be used as both a greeting and as a farewell in Spanish.
Since you are greeting someone at 8 PM, you would say buenas noches.
Answer:
Buenas noches