The nundinae (nənˈdɪnaɪ, -niː), sometimes anglicized to nundines, were the market days of the ancient Roman calendar, forming a kind of weekend including, for a certain period, rest from work for the ruling class (patricians).
The nundinal cycle, market week, or 8-day week (nundinum or internundinum) was the cycle of days preceding and including each nundinae. These were marked on fasti using nundinal letters from A to H. The earliest form of the Roman calendar is sometimes said to have included exactly 38 such cycles, running for 304 days from March to December before an unorganized expanse of about 50 winter days. The lengths of the Republican and Julian calendars, however, were not evenly divisible by 8; under these systems, the nundinae fell on a different letter each year. These letters formed the basis of the later Christian dominical letters.
The name nūndinae (ˈnuːn.dɪ.nae̯) was apparently formed from an early form of nōnus ("ninth") and -din- ("day"), a root related to diēs and ultimately the Proto-Indo-European root reconstructed as *dyew- ("to shine"). It is now glossed as an adjective modifying an understood feriae ("festival; holiday"), but not all Romans considered it to be one: a writer named Titius listed the nundinae as a "customary occasion" (sollemnes) and the Roman pontiffs themselves told the augur Messala that they did not consider the nones or nundinae to be religious occasions. Like feriae and the names of most other recurring days of the Roman calendar, nundinae always appeared as a plural in classical Latin, even in references to a single day. The English form "nundine", following French nundines, similarly appeared only as a plural at first, although it is now used in the singular number for individual days. In Roman inscriptions, the word was abbreviated .
The form nundinum for the span between the nundinae seems to have been standard in early Latin, but only appears in compounds (internundinum, trinundinum, &c.) and phrases (inter nundinum) in the classical period.