long month; month with 31 days
Refers to a month that has 31 days. Often contrasted with 小の月 (short month, 30 days or less). Used in everyday contexts like calendars and scheduling.
See also: 小の月
January is a long month (has 31 days).
There is a song to remember the long months and short months.
Compound of 大 (だい, 'big, large') + の (possessive particle) + 月 (つき, 'month'). Literally 'big month', referring to months with more days.