Nundinal cycle

The Roman calendar changed its form several times between the founding of Rome and the fall of the Roman Empire. The common calendar widely used today is known as the Gregorian calendar and is a refinement of the Julian calendar where the average length of the year has been adjusted from 365.25 days to 365.2425 days (a 0.002% change).

From at least the period of Augustus on, calendars were often inscribed in stone and displayed publicly. Such calendars are called fasti.

The original Roman calendar is believed to have been a lunar calendar, which may have been based on one of the Greek lunar calendars. As the time between new moons averages 29.5 days, its months were constructed to be either hollow (29 days) or full (30 days).

Roman writers attributed the original Roman calendar to Romulus, the mythical founder of Rome, though there is no other evidence for the existence of such a calendar and Romulus was often cited as the founder of practices whose origins were unknown to later Romans. According to these writers, Romulus' calendar had ten months with the spring equinox in the first month, a theory probably based on the names of the last six months of the year:

The regular calendar year thus consisted of 304 days (38 nundinal cycles), with the winter days after the end of December and before the beginning of the following March not being assigned to any month.

The origins of the names are also not entirely clear or agreed upon by modern scholars. Some ancient explanations are: Martius in honour of Mars, the god of war; Aprilis from aperiō, to open: Earth opens to receive seed; Maius from Maia, goddess of growth (maior, elder); Iunius from iunior (younger). The remaining six months were named with respect to their position on the calendar: the numbers five to ten in Latin being quinque, sex, septem, octo, novem and decem, the months were named Quintilis, Sextilis, September, October, November, and December.

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