To say "good evening" in Spanish, you can say: "Buenas noches." This phrase is commonly used both as a greeting in the evening and as a way to say good night. If you want to be more formal or polite, you can say:

• "Buenas tardes" (Good afternoon) — typically used until around sunset.
• "Buenas noches" (Good evening / Good night) — used after sunset.

So, for evening greetings, "Buenas noches" is the most appropriate and widely used phrase.