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The remnants of this early calendar still can be recognized in the numbered names for Quinctilis (July), Sextilis (August), September, October, November, and December.
The two months of winter, when there was no work in the fields, were not counted; Cato, for example, speaks of payment for olives being due in ten months (De Agricultura, CXLVI).
Nor did the college of pontiffs (from pontifex or "bridge maker"), who were responsible for regulating the calendar and the festivals that depended upon it, always intercalate the additional days necessary to synchronize the lunar and solar year.
Intercalation was considered unlucky and, during the Second Punic War (218-201 BC), when Rome struggled against Carthage, the priests were hesitant to make any changes at all.
(The notion of February 29 is a modern construct.) When Caesar was assassinated in 44 BC, Quinctilis, the month in which he had been born, was renamed Julius (July) in his honor, although this change in the name of the month was ignored until made legal after the appearance of a comet four months later during games sponsored in July by Octavian, which, recounts Cassius Dio, was understood to be a sign of Caesar's apotheosis. Could anything be more unseemly than 'July' for Brutus?
Octavian is thought to have moved these games to late July to overshadow games sponsored by Brutus earlier that month. " Cicero also joked in a letter just four days after the new calendar had been introduced that the constellation of Lyre was rising "by Caesar's decree." But the pontiffs mistakenly adjusted for leap year every three years (having counted inclusively) and inserted too many intercalary days.
Beginning in March in the spring and ending in December with the autumn planting, the year then was ten months long and had six months of thirty days and four of thirty-one, for a total of 304 days (ten lunar months actually comprise about 295 days).
Since each month began and ended with the new moon, that day would have belonged both to the new month and the old and must have been counted twice.
Only then, after the superfluous days had been corrected and intercalation was resumed, did the Julian calendar function as intended, with February gaining an extra day every four years.
Considered unlucky, it was devoted to rites of purification (februa) and expiation appropriate to the last month of the year.
(Although these legendary beginnings attest to the venerability of the lunar calendar of the Roman Republic, its historical origin probably was the publication of a revised calendar by the Decemviri in 450 BC as part of the Twelve Tables, Rome's first code of law.) But the moon's lunation, a period of approximately 29.5 days, is not the same length of time as the earth's annual orbit around the sun, a period of approximately 365.25 days.
According to Livy (I.19), it was Numa Pompilius, the second king of Rome (715-673 BC), who divided the year into twelve lunar months.
Fifty days, says Censorinus (XX), were added to the calendar and a day taken from each month of thirty days to provide for the two winter months: Januarius (January) and Februarius (February), both of which had twenty-eight days.
Bibulus, Caesar's co-consul in 59 BC, for example, attempted to thwart the legislation of his more powerful colleague by declaring all the remaining days of the year to be holidays so the assembly could not legally meet (Dio, XXXVIII.6.1).