"What day of the week is tomorrow?"
Translation:ʻO ka Pōʻahia ka lā ʻapōpō?
I think the key is thinking in terms of definition (ʻO) vs state or location (Aia). “Tomorrow” is not located on the day after today the way, say, a party would be:
Aia ka pāʻina ma ka lā ʻapōpō.
Rather, we are talking about the definition of tomorrow, which (based on what today is) is defined as a particular day.
Lecia, I feel your frustration. I left quite a lengthy comment to the Duolingo folks a week ago concerning the format of these Hawaiian lessons. The lessons are better suited at testing rather than teaching. There are quite a few of us frustrated with this course in general because we're not being taught. Rather we're learning by failure which is not the way to do it. I hope they consider my comments and yours as well.