"Is this Saturday the ninth?"
The English is ambiguous though - it can mean "is this day Saturday the 9th", or "is it the 9th on this Saturday". The former you might use if somebody was talking about a day or an event and you wanted to clarify they meant "Saturday the 9th". I don't know how you'd ask that in Japanese - I tried これは土曜日9日なの？ but not accepted.
It seems like Duolingo could come up with a way to use the readings of the kanji selected for the full sentence in the tiles. I realize the better option is to just enable writing mode, but some folks may find it extremely useful to hear the correct reading when clicking the tiles.
I agree with other comments that this one is ambiguous, but I get what it's trying to say: "Is this coming Saturday (or this week's Saturday) the 9th?"
However, if you were trying to say "Is this (as in, today) Saturday the 9th?", would that be "これは九日の土曜日ですか？" or "今日は九日の土曜日ですか？" Or would it be said another way entirely?