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.
Yes it must be in accusative case cause it is originally:
أتمنى لك يوماً سعيداً
Like the expression "welcome":
" أهلاً و سهلاً "
Wich is originally:
وطئتم أهلاً و حللتم سهلاً
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.
Edit: Jad498154 is correct. The correct spelling is "يومكَ/كِ".
يومكَ/كِ و ليس كي
"Good day!" should be accepted.
A happy day should be correct as well, right?
Have a nice day
Where is that said? Not anywhere I am aware of.
Another native arabic speaker here, and I have never heard anyone say this in my life...
Who wanna learn arabic co tact me my istagram @najib_h.s.n