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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, instituted according to Roman tradition by Numa Pompilius, was supposed to be inserted every two years or so to align the conventional 355-day Roman year with the solar 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, who would normally be an active politician, often manipulated the decision to allow friends to stay in office longer or force enemies out early. Such unpredictable intercalation meant that dates following the month of Februarius could not be known in advance; and Roman citizens living outside Rome would often not know the current date.
The exact mechanism is not clearly specified in ancient sources. Some scholars, such as Ludwig Ideler, Henry G Liddell, 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. This view is followed in generalist surveys of calendrical history such as those of D E Duncan, G R Richards or A Aveni.
However, following a 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  claim that in intercalary years Februarius was set at either 23 or 24 days, and followed by an intercalary month of 27 days. Whichever interpretation is correct, the days a.d. VI Kal. Mart. to Prid. Kal. Mart., normally referring to the end of February, were in intercalary years the concluding days of the mensis intercalaris.
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 .
- Henry G Liddell, 1909, A History of Rome, John Murray, London, p. 29
- "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.
- Liddell, 1909, A History of Rome, John Murray, London, p. 29
- E J Bickerman, Chronology of the Ancient World, Ithaca, New York 1980, ISBN 0-80-141282-X.
- 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
- The view is opposed by H. Chantraine, whose opinion is in turn dismissed by Brind'Amour as special pleading
- 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.