It is a quirk of duolingo to be unable to accept all correct answers. The difficult thing is that they are not consistent about how they do that. For this particular translation, they are using the most common translation of IU's song, i think.
"Such a good day" should be accepted. Not only is that natural in English, but "좋다" means "to be good". "좋은" literally means "good" in English. How could that not be one of the answers?
No offense, but oversights like this infuriate the users.