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'Mercedonius', also known as 'Intercalaris', was the intercalary month added in leap years of the Roman calendar. It had 27 days, beginning after the 23rd or 24th day of Februarius, so that the year was lengthened by a total of 22 days. Romans believed that the month had been added to the Roman calendar, along with Januarius 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.
This month was supposed to be inserted every two or three years to align the 355-day common year with the tropical year.^[1] 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.^[2] 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; neither could the current date for citizens out of communication with the city.
The month was eliminated by Julius Caesar when he introduced the Julian calendar in 46 BC.

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Notes

1. "The lunar year of 354 days fell short of the solar year by 11-1/4 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"
D. Spillan, Livy's ''History of Rome'', Book I. 19. Footnote 24.
This is the theory of Macrobius in ''Saturnalia'' (c. AD 430).
2. "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.

See also

★ Julian calendar: Motivation

External link

★ Roman Dates