December (from Latin decem, "ten") or mensis December was originally the tenth month of the Roman calendar, following November (novem, "nine") and preceding Ianuarius.
When the calendar was reformed to create a 12-month year starting in Ianuarius, December became the twelfth month, but retained its name, as did the other numbered months from Quintilis (July) to December.
Ian., "the 9th day before the Kalends of Ianuarius," on the Julian calendar (VII Kal.
On the calendar of the Roman Republic and early Principate, each day was marked with a letter to denote its religiously lawful status.
Each day was marked with a letter such as:[2] By the late 2nd century AD, extant calendars no longer showed days marked with these letters, probably in part as a result of calendar reforms undertaken by Marcus Aurelius.