"What day is your birthday?"
Translation:ʻO ka Pōʻahia kou lā hānau?
I do not quite understand the question, but I translate what you wrote as:
Aia ma ka lā ʻehia kou lā hānau? - What date is your birthday on? Or What day of the month is your birthday on?
Aia ma ka lā ʻehia ka pāʻani pōpeku? - What date is the football game on? Or What day of the month is the football game on?
This "day/date" distinction is very difficult. Usually when one asks about a birthday, one expects a day of the month answer. But actually, one would probably say "When," not "What day." I thought I had learned my lesson with "What day is the holiday," but it turns out they wanted ka lā 'ehia!