Words that count days use an exceptional reading. Even though this is a jukugo word, it uses kun'yomi readings for both of the kanji. You learned this reading for 九 when you learned 九つ, so you should be able to read this one too!
Would you like to go watch some soccer with me on the ninth?
Since Koichi hasn't seen Nikka for nine days, he feels so lonely he could die.
Mami’s birthday is April ninth.