Translation:Is this Saturday the ninth?
Just add 目（め） to the regular counter: 九つ目（ここのつめ）
Alternately, if you want to indicate "number 9", use 番（ばん）: 九番（きゅうばん）
...And again, you can add 目 to indicate "ninth": 九番目（きゅうばんめ）
More info about Japanese numbers/counting:
Flip, and then I remembered that all squares are rectangles, but not all rectangles are squares (just because A can be B doesn't mean B can be A). What I said only works with math and equal signs, which then suggests why OP's sentence doesn't work...
Yay for mobile not having editing powers or deleting powers...
Is this Saturday 9th? Rejected.
In English, it's usual to write dates with digits, not as words - and we write "the ninth" as "9th", not as "the 9th".
I've just failed to test out because I had four questions where either the so-called "correct" answer was in unnatural English, or the "correct" answer was okay but my answer was also okay and was rejected. This is really frustrating.