月

月 (yuè) means both 'moon' and 'month' — the same character, because the lunar cycle is what originally defined the month in Chinese. In modern use, the two meanings rarely conflict: context decides. For months of the year, 月 attaches directly to a number: 一月 (January), 二月 (February), 三月 (March), all the way to 十二月 (December). For duration ('three months'), you need 个: 三个月 (sān ge yuè). Two things English speakers should note: first, the difference between 三月 ('March,' a specific month) and 三个月 ('three months,' a length of time) is a beginner trap — drop or add 个 and the meaning flips; second, for 'moon' as a celestial body, modern speech almost always uses 月亮 (yuèliang) rather than 月 alone, which survives mainly in compounds and poetry.

Memory hook: 月 is a stylized crescent moon. One moon cycle = one month. Same character does both jobs.