short month; month with fewer than 31 days
Refers to months with 30 or fewer days (February, April, June, September, November). Often contrasted with 大の月 (long month, 31 days).
See also: 大の月
For short months, you need to be careful with the dates at the end of the month.
February is a short month, with only 28 or 29 days.
Compound of 小 (small) + の (possessive particle) + 月 (month), literally 'small month'.