Mercedonius
Encyclopedia
Mercedonius, also known as Intercalaris, was the intercalary month added in leap year
Leap year
A leap year is a year containing one extra day in order to keep the calendar year synchronized with the astronomical or seasonal year...

s of the Roman calendar
Roman calendar
The Roman calendar changed its form several times in the time between the founding of Rome and the fall of the Roman Empire. This article generally discusses the early Roman or pre-Julian calendars...

. The resulting year was either 377 or 378 days long. The exact mechanism by which this was done is not clearly specified in ancient sources. Modern scholarship holds that Februarius
February
February is the second month of the year in the Julian and Gregorian calendars. It is the shortest month and the only month with fewer than 30 days. The month has 28 days in common years and 29 days in leap years...

 was truncated to 23 or 24 days, to be followed by an intercalary month of 27 days. Earlier scholars such as Ideler held the view that in intercalary years February's length was fixed at 23 days and it was followed by a variable length mensis intercalaris with 27 or 28 days. Whatever the details, the five days a.d. VI Kal. Mart. to Prid. Kal. Mart., normally located at the end of February, are the concluding days of the mensis intercalaris in intercalary years. As a result, the year was lengthened by a total of 22 or 23 days. Romans believed that the month had been added to the Roman calendar, along with Ianuarius
January
January is the first month of the year in the Julian and Gregorian calendars and one of seven months with the length of 31 days. The first day of the month is known as New Year's Day...

 and Februarius, by King Numa Pompilius
Numa Pompilius
Numa Pompilius was the legendary second king of Rome, succeeding Romulus. What tales are descended to us about him come from Valerius Antias, an author from the early part of the 1st century BC known through limited mentions of later authors , Dionysius of Halicarnassus circa 60BC-...

 in the 7th century BC. The name Mercedonius comes from merces, meaning wages, as workers were paid at that time of year.

This month was supposed to be inserted every two or three years to align the 355-day common year with the tropical year
Tropical year
A tropical year , for general purposes, is the length of time that the Sun takes to return to the same position in the cycle of seasons, as seen from Earth; for example, the time from vernal equinox to vernal equinox, or from summer solstice to summer solstice...

. The decision whether to insert the intercalary month was made by the pontifex maximus
Pontifex Maximus
The Pontifex Maximus was the high priest of the College of Pontiffs in ancient Rome. This was the most important position in the ancient Roman religion, open only to patricians until 254 BC, when a plebeian first occupied this post...

, supposedly based on observations to ensure the best possible correspondence with the season
Season
A season is a division of the year, marked by changes in weather, ecology, and hours of daylight.Seasons result from the yearly revolution of the Earth around the Sun and the tilt of the Earth's axis relative to the plane of revolution...

s. Unfortunately the pontifex maximus (whose office was generally held by a politician or soldier, notably Julius Caesar
Julius Caesar
Gaius Julius Caesar was a Roman general and statesman and a distinguished writer of Latin prose. He played a critical role in the gradual transformation of the Roman Republic into the Roman Empire....

 during the so-called Years of Confusion) often neglected to insert the month at the proper time, or deliberately inserted it early or late to allow some officials to stay in office longer or force others out early. Such unpredictable intercalation meant that dates following Februarius could not be known in advance; and Roman citizens living outside of Rome would often not know the current date.

The month was eliminated by Julius Caesar when he introduced the Julian calendar
Julian calendar
The Julian calendar began in 45 BC as a reform of the Roman calendar by Julius Caesar. It was chosen after consultation with the astronomer Sosigenes of Alexandria and was probably designed to approximate the tropical year .The Julian calendar has a regular year of 365 days divided into 12 months...

in 46 BC.

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