The Roman calendar was a dating system purportedly enacted by Romulus, the cofounder of Rome, in the year 738 BCE. Originally, the calendar had 10 months and 304 total days. Six calendar months contained 30 days and 4 calendar months contained 31 days. The remaining 61 days that make up our current tropical calendar of 365 days were at that time ignored, falling into an uncounted winter season.
The 10 months of the Roman calendar were called Martinus, Aprilis, Maius, Janius, Quintilis, Sextiles, September, October, November, and December. Numa Pompilius, the second king of Rome, inserted the month of January at the beginning of the year and the month of February at the end of the year. In 452 BCE, February was relocated to its current calendar position between January and March.
Despite the addition of January and February, the calendar was still a full 10.25 days short of the current 365-day calendar year. This shortage caused confusion in seasons and lunar cycles, and time became endlessly out of sync. In an attempt to prevent further misalignment of the calendar and the seasons, an occasional intercalation was performed, allowing for the addition of a 27-28 day month every 2 years. This created an average of 366 days per year when extrapolated every 4 years, and it kept the seasons on track. This practice, also called Mercedonius, comes from the term merces, meaning wages, because workers were paid at this time of the year.
This intercalation method continued to cause confusion. Intercalation was an official duty of pontifices, and their reasons for timing of the intercalations often remained secret. Calendars were not public documents, and political tactics to lengthen or elongate the ruling terms of magistrates and other public officials confounded the problems.
In an effort to construct a more sound and incorruptible calendar system, Julius Caesar initiated a calendar reform. In 46 BCE, the Julian calendar was established; it was later modified into the Gregorian calendar now in general use.
See also Gaius Julius Caesar; Julian Calendar; Gregorian Calendar; Measurements of Time
Feeney, D. C. (2007). Caesar’s calendar: Ancient time and the beginnings of history. Berkeley: University of California Press.
Michels, A. K. (1967). The calendar of the Roman republic. Princeton, NJ: Princeton University Press.