Finally, I can be an Esperantist gameshow host.
Welcome to my Lair, and let the games begin: MWAHAHAHA
Duo: "Good evening and welcome!"
Me: "No, please, Duo, I'll learn more useful phrases!"
Why is this bonan and not bona? Is it assumed to be "[I wish you a] good evening"?
Why not: Good afternoon? Guten Tag is till what time?
"Vespero" specifically means "evening." The best translation of "Guten Tag/Good day" in Esperanto would be "Bonan tagon."
Guten Tag is acceptable at all times, except perhaps evening, or night.