Why is it 'una buena tarde' and 'unas buenas noches'?

Why is it ‘una buena tarde’ and ‘unas buenas noches’? In the second one a plural? Thanks :slight_smile:

¡Sí! :slight_smile:

Here is the answer (the explanation) Hope it will help
http://www.hispanoteca.eu/Foro-preguntas/ARCHIVO-Foro/Buenos%20días-buenas%20tardes-noches.htm