Home > Calendar > Calendar Months

Months of the Year

The months of the year originated as a way to mark time and break up the long seasons into shorter periods based on the Moon’s cycle around the Earth.

A month is a unit of time that was first used and invented in Mesopotamia to measure the natural period related to the cycle of the moon.

Our modern Gregorian calendar has twelve months, of either 28, 30 or 31 days during the common year. Nearly every 4 years is a Leap Year when an extra (intercalary) day is added at the end of February, the Leap Day on 29 February.

The Gregorian and the Julian calendar both consist of twelve months. The original Roman calendar consisted of 10 named months and two unnamed months. The year also began in March or “Martius” because this was the time when spring began. The months of January (Januarius) and February (Februarius) were added to the calendar during the Julian calendar reform around 700 BCE by Numa Pompiliius, the second king of Rome.  January also became the starting month of the year.

The addition of January and February moved the months September, October, November and December to no longer correspond with their meaning. The months from September through December have names that mean the seventh through tenth months of the Roman calendar.

There are many calendars that use months to divide up the year such as the Islamic calendar, the Hebrew calendar, and the Hindu calendar to just name a few. Although the Gregorian calendar is the most commonly used calendar today, other calendars are still used to calculate certain holidays and annual feasts to correspond with the Gregorian calendar.

Months Used in the Past

Months that existed in the Roman calendar that are no longer used include:

The Twelve Months of the Year

The Gregorian calendar and the Julian calendar both consist of the following twelve months:

The Gregorian calendar has four months with a length of 30 days and seven months that are 31 days long. February is the only month that is 28 days long in common years and 29 days long in leap years.


Other calendars

More information

Calendar tools