month

A month is a unit of time used in most civil calendars, roughly based on the cycle of the Moon or on a division of the year into a set number of days. The term derives from the word for Moon in many languages, reflecting historic association with the Moon’s changing visible disk.

Two astronomical month lengths are commonly distinguished. The synodic month, about 29.5306 days, measures the Moon’s phases from new Moon to new Moon. The sidereal month, about 27.3217 days, is the Moon’s orbital period relative to the background stars. In calendar systems, months are assigned fixed numbers of days, typically 28 to 31.

In the Gregorian calendar used widely today, there are 12 months. January and March, May, July, August, October, and December have 31 days; April, June, September, and November have 30; February has 28 days in common years and 29 in leap years. Leap years keep the calendar roughly aligned with the tropical year.

Other calendars use different month schemes. Purely lunar calendars, such as the Islamic calendar, have 12 lunar months totaling 354 or 355 days, so months drift through the seasons. Lunisolar calendars, like the Hebrew and traditional Chinese calendars, insert leap months to keep months roughly aligned with the solar year.

Months are a fundamental unit for timekeeping, finance, and scheduling, used to organize weeks, quarters, and fiscal years. The concept is universal, though exact lengths and naming vary by culture and calendar system.