Hi Taylor, I totally agree with Beatriz. With friends I'd use "buenos días (morning)/buenas tardes (between lunch and dinner times)/buenas noches (after dinner)" or just "buenas" for the whole day. In fact I mainly use "buenas" and "hola" (even on formal situations). On formal situations or with clients at work, you can use "que pases/tengas un buen día/buena tarde" "que pase/tenga un buen día/buena tarde (even more formal)" to say goodbye but I'd only use it on phone calls, if anything. My feeling is that in Spain we are going "less formal" everyday. I'd forget about "usted" as I never use it (in any situation) but it's totally up to you. FYI I'm working at a very important rubber company and I deal with very formal situations and I hardly ever hear "usted". I hope it helps.

Hola Taylor. Aquí te dejo mis respuestas: 1. "Buenos días" o "buenas tardes" lo usamos para saludar o empezar una conversación. 2." Que tenga/s un buen día o "que tenga/s una buena tarde" lo usamos para terminar una conversación. Normalmente lo usamos cuando hablamos con clientes o con superiores en un entorno profesional o cuando hablamos con personas muy mayores como acto de cortesía. Con amigos yo no lo uso nunca. Saludos.

You're rigth. Normally in spanish you say "Buenas tardes" , yes, in plural , even if u are refering to an especific day ,It's normal! Im Venezuelan and the first lenguage is Spanish , so I'm pretty sure about my opinion:)

Que tenga una buena noche

My natal lenguage is Spanish and the people often say "buenas tardes" and if somebody hear that, it will be normal, but the people that are polite or know about grammar say "que tenga buena tarde" because you are talking about the specific day that it is happening and for that reason is in singular because is just an evening of one day not a lot of days, so the best form to say that is "que tenga buena tarde" I'm sorry if I have some mistakes but I'm learning English.