Months

Months are a unit of time used to subdivide a year in most calendar systems. In the Gregorian calendar, which is widely used today, there are 12 months. Their lengths vary: seven months have 31 days, four have 30 days, and February has 28 days in common years and 29 in leap years. A standard year has 365 days, and a leap year has 366 days.

Most month names in English derive from Latin. January is named after the god Janus, February from Februa, March from Mars, May from Maia, June from Juno, July from Julius Caesar, and August from Augustus. The names September through December come from the Latin numbers seven to ten, reflecting an older counting system when the year began in March. The current order, with January as the first month, shifts relative to its ancient roots due to calendar reforms.

History: The early Roman calendar had 10 months; winter was unassigned. January and February were added later. The Julian calendar of 45 BCE standardized 12 months and a 365-day year with leap years every four years. The Gregorian reform of 1582 corrected drift in the equinoxes by skipping 10 days and refined leap-year rules: a year divisible by 4 is a leap year, except centuries not divisible by 400. This yields a 400-year cycle that keeps the calendar year aligned with the solar year.

Other calendars: Lunar calendars and lunisolar calendars use lunar months; the Islamic calendar uses 12 lunar months of 29 or 30 days and shifts relative to the solar year. The Hebrew calendar uses 12 months in common years with occasional intercalary months to realign with the solar year. In many cultures, months are used for agriculture, religious observances, and fiscal planning, and fiscal months may not align exactly with calendar months.