Translation:Have a nice day!
I've never heard this said by Arabic speakers, and I've certainly never said it in my life. I knew "Happy day!" was meaningless, but that was my answer, and it wasn't accepted.
"Happy day" should definitely be accepted, report it. This is not something I've ever heard an Arab say either, and even if it theoritically were, we would have used the accusative and said "يومًا سعيدًا" and not the nominative.
I think someone thought, "How would you say 'Have a nice day' in Arabic?" Well, you just wouldn't, because it's not an Arabic expression.
I'm a native Arabic speaker, and I've never used or heard such a phrase in my life. I would suggest "طاب يومك/ي", as a more acceptable translation.