Many Spanish greetings depend on the time of day. Even though Buenos días includes the word day, it's only used in the morning. Buenas noches can mean both Good evening and Good night.
Spanish pronunciation is easier than you think! In English, the same letter can sound very different depending on the word (such as the a in hat and hate). But in Spanish, letters have consistent pronunciations. For example, the a in adiós sounds the same as the a in cama.