Etymology: Originally written showing a bent elbow, which was the sign for the number nine when counting with only one arm.
Hint: Think of this as ten with a hook on the cross stroke meaning "minus 1" - thus nine.
Mnemonic: LESS THAN PERFECT TEN = NINE
九日 (kokonoka) the ninth day
九月 (kugatsu) September
(Based upon Henshall, 1998)