The Roman calendar was the calendar used by the Roman Kingdom and Roman Republic. Although primarily used of Rome's pre-Julian calendars, the term often includes the Julian calendar established by the reforms of the dictator Julius Caesar and emperor Augustus in the late 1st century BC.

The original calendar consisted of ten months beginning in spring with March; winter was left as an unassigned span of days. These months each had 30 or 31 days, and ran for 38 nundinal cycles, each forming an eight-day week (nine days counted inclusively in the Roman manner, hence the name) ended by religious rituals and a public market. The winter period was later divided into two months, January and February. The legendary early kings Romulus and Numa Pompilius were traditionally credited with establishing this early fixed calendar, which bears traces of its origin as an observational lunar one. In particular, the kalends, nones, and ides of the month seem to have derived from the first sighting of the crescent moon, the first-quarter moon, and the full moon respectively. The system ran well short of the solar year, and it needed constant intercalation to keep religious festivals and other activities in their proper seasons. This is a typical element of lunisolar calendars.

After the establishment of the Republic, years began to be dated by consulships and control over intercalation was granted to the pontifices, who eventually abused their power by lengthening years controlled by their political allies and shortening the years in their rivals' terms of office. Having won his war with Pompey, Caesar used his position as Rome's chief pontiff to enact a calendar reform in 46 BC, coincidentally making the year of his third consulship last for 446 days. In order to avoid interfering with Rome's religious ceremonies, the reform added all its days towards the ends of months and did not adjust any nones or ides, even in months which came to have 31 days. The Julian calendar was supposed to have a single leap day on 24 February (a doubled VI Kal. Mart. or ante diem bis sextum Kalendas Martias) every fourth year, but following Caesar's assassination the priests figured this using inclusive counting and mistakenly added this bissextile (bis sextum) day every three years. In order to bring the calendar back to its proper place, Augustus was obliged to suspend intercalation for one or two decades. The revised calendar remained slightly longer than the solar year; by the 16th century the date of Easter had shifted so far away from the vernal equinox that Pope Gregory XIII ordered the calendar's adjustment, resulting in the Gregorian calendar.