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The 'mensis intercalaris' was the intercalary month added in leap years of the Roman calendar. The resulting year was either 377 or 378 days long. According to the first-century AD writer Plutarch the month was also known as Mercedinus or Mercedonius.
This month was supposed to be inserted every two or three years to align the 355-day common year with the tropical year. The decision whether to insert the intercalary month was made by the pontifex maximus, supposedly based on observations to ensure the best possible correspondence with the seasons. Unfortunately the pontifex maximus (whose office was generally held by a politician or soldier, notably Julius Caesar 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 exact mechanism by which this was done is not clearly specified in ancient sources. Some scholars, such as Ludwig Ideler, the staff writers of Encyclopaedia Britannica and Elias Bickerman hold 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. The last two works are the specialist treatments most available to the general public. This view is followed in generalist surveys of calendrical history such as those of D E Duncan, G R Richards or A Aveni. Following the discussion of intercalation in A. K. Michels, The Calendar of the Roman Republic (Princeton, 1967) 145–172, the standard reference on the pre-Julian calendar, some specialist studies of the pre-Julian calendar published since 1967 including papers and books by A. E. Samuel, P. S. Derow, P. Brind'Amour, V. M. Warrior, J. Rüpke, R. Hannah, and C. J. Bennett claim that in intercalary years Februarius was truncated to 23 or 24 days, to be followed by an intercalary month of 27 days. Against: H. Chantraine, dismissed by Brind'Amour as special pleading. 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 and Februarius, by King Numa Pompilius in the 7th century BC. The name Mercedonius comes from merces, meaning wages, as workers were paid at that time of year.
The month was eliminated by Julius Caesar when he introduced the Julian calendar in 46 BC.
- Plutarch, Life of Numa XVIII.2, available at .
- Plutarch, Life of Caesar LIX.2, available at .
- "The lunar year of 354 days fell short of the solar year by 111⁄4 days;—this in 8 years amounted to 90 days. These 90 days he divided into 2 months of 22 and 2 of 23 days, and introduced them alternately every second year for two octennial periods: every third octennial period, however, Numa intercalated only [...] 3 months [...] because he adopted 355 days as the length of his lunar year"
Daniel Spillan, Livy's History of Rome, Book I. 19. Footnote 24.
This is the theory of Macrobius in Saturnalia (c. AD 430).
- "Their management was left to the pontiffs—ad metam eandem solis unde orsi essent—dies congruerent; 'that the days might correspond to the same starting-point of the sun in the heavens whence they had set out.'"
D. Spillan, Livy's History of Rome, Book I. 19. Footnote 24.
- C Ludwig Ideler, Handbuch der mathematischen und technische Chronologie, Berlin 1825.
- E J Bickerman, Chronology of the Ancient World, Ithaca, New York 1980, ISBN 0-80-141282-X.
- Some of these writers assume that the various extracts from the Roman jurist Celsus (Digest volume 39) quoted in the Significations [Definitions] of Justinian's Law Code (The Enactments of Justinian, The Digest or Pandects, tr. S P Scott, Cincinnati 1932 available at ) develop an argument. This is not the case in Book 50, which is a series of unrelated dictionary definitions.