The Ancient Roman Calendar: History, Months & Saints

Instructor: Christopher Cudabac

Chris has taught World History and Cultures at the High School level and has a master's degree in Classics.

In this lesson, you will learn how the calendar of the ancient Romans worked, how it evolved as a part of Roman politics and culture, and how to navigate some of its unfamiliar and counter-intuitive aspects.


When we modern people study the Roman calendar, we may find its arrangement of days to be absurd. But we can definitely find familiar elements, such as the names of the months. In this lesson, we'll trace the characteristics and history of the Roman calendar, going through its designations for the days, weeks, and months.

Counting the Days

The earliest Roman calendar was based on cycles of the moon, which influenced their method for designating days in a month. The Romans counted down the days to one of three special days: the kalends, nones, and ides. The kalends were the first of the month, coinciding with the new moon; the nones were at the first quarter, and the ides were at the full moon. Later, when the months were disassociated from the lunar cycles, these days were fixed: the kalends on the first of the month, the nones usually on the fifth, and the ides usually on the thirteenth.

Each day in the month was described one of two ways: either as on one of these days ('on the nones of April' was April 5), or by counting down inclusively to the next special day. For example, April 3 was 'three days before the nones of April' (count April 5, April 4, April 3). After the ides had passed, the countdown looked forward to the kalends of the next month: April 14 was, for a Roman, 18 days before the kalends of May.

Days in a Week

Within the month, the Romans used an eight day week that ended in a market day, nunidia. Later, this arrangement lost popularity to the present seven day week.

Above, on this fragmentary Roman calendar, the letters on the left count to the nunidia (H). The numerals count down to the kalends of the next month. The abbreviations on the right show the nunidia (N) and what government business can be done on each day; the smaller text indicates festivals.
ancient Roman calendar

Of Moons and Months

Earliest Roman calendars had ten lunar months, starting with Martius (March), which was the start of the campaigning season and thus sacred to Mars, the Roman war god. Origins for the names of April, May, and June are now unclear. Names for other months came from Latin numbers; for example, decem, 'ten,' provided the name for December, the tenth month of this calendar. About sixty days of winter were left without any designation.

At an early period the Romans formed two months from these nondescript days in winter: January, which was holy to Janus, the god of entrances, and February, which took its name from the Latin verb for purification and atonement. The lunar cycle was dropped, and most months alternated between 29 or 31 days; because of its gloomy associations, February was set at 28 days, providing a year of 355 days.

In the second century BCE, the Romans moved the start of the year to January, so that the magistrates, who assumed office at the year's beginning, could get to far-flung armies before the start of the campaigning season.

Measuring the Year

Even the earliest Romans knew that the solar year was longer than 355 days, so the the chief priest of Rome and keeper the official calendar, the pontifex maximus, could declare at his discretion a short intercalary month to keep the year in step with the seasons. But these pontifices could also abuse their absolute calendar-keeping authority by decreeing the date to be one on which no public business could be done, throwing the calendar out of step. By the first century BCE, the calendar trailed the solar year by more than 67 days!

When Julius Caesar was elected pontifex maximus--about the same time he was declared dictator for life in 45 BCE--he had less need for calendar trickery and was open to major calendar reforms. He changed the lengths of most months to alternate between 30 and 31 days, kept February at 28 days, and so established the familiar year of 365 days. From his time in Egypt (remember Cleopatra?), he knew that astronomers there had observed the solar year to be 365 ¼ days. So, to make up for the day lost every four years, he established that a day would be added to February every fourth year, which would be later known as 'leap year.'

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