Learn Italian: Arrivederci e buona giornata, dottore! - Goodbye and have a nice day, doctor!

Both mean "good day," but they are used differently. You say buongiorno when you greet someone (hello or good morning). You say buona giornata when you are leaving, to wish them a good day. Since this sentence starts with arrivederci (goodbye), buona giornata is the perfect fit.

Why use arrivederci instead of ciao?

Ciao is informal, used with friends and family. When speaking to a professional like a doctor, you should use a more polite goodbye. Arrivederci is the standard, polite way to say goodbye to someone you address formally.

Does dottore only mean a medical doctor?

Not in Italy! While it does mean a medical doctor, dottore (or dottoressa for a woman) is also used as a polite title for anyone who has a university degree. It is very common to hear it used in everyday professional settings as a sign of respect.

Why isn't there a word for "the" before dottore?

When you are speaking directly to someone using their title, you drop the definite article (the word for "the"). If you were talking about the doctor to someone else, you would say il dottore, but since you are addressing him directly here, you just say dottore.