Many Spanish greetings(Saludos) depend on the time of day.

Buenos dias – Good morning

Note that even though Buenos dias includes the word dia, it’s only used in the morning.

Buenas tardes – Good afternoon / Good evening

You can use Buenas tardes in the afternoon or in the early evening.

Buenas noches – Good evening / Good night

Buenas noches can mean both Good evening and Good night.