noun
The ninth month of the Gregorian calendar.
九月になったら涼しくなるでしょう。
It will probably get cooler in September.
noun
ninth month of the lunar calendar
Refers to the ninth month of the traditional Japanese lunar calendar, roughly corresponding to October in the modern calendar. Rare in everyday use; mainly encountered in historical or literary contexts.
旧暦の九月は長月とも呼ばれる。
The ninth month of the lunar calendar is also called Nagatsuki.