Roman calendar

The Roman calendar was used by the Roman Kingdom and Roman Republic. It began with a simple system created by Romulus, who is said to have been Rome's first king. This early calendar had ten months, starting with March and ending with October, leaving the winter days unmarked. Each month had either 30 or 31 days and was organized into eight-day cycles called nundinal cycles, ending with markets and religious events.

Later, Romulus's successor, Numa Pompilius, added two more months: January and February, to cover the winter season. He also tried to align the calendar with the sun's yearly path by adding extra days in a process called intercalation. However, keeping the calendar correct was difficult, and over time it fell out of sync with the seasons.

Finally, Julius Caesar changed the calendar completely in 46 BC. He created the Julian calendar, which was based on the sun's path and added a leap day every four years to keep things balanced. This system was used for many years and later became the basis for the calendar we use today, the Gregorian calendar, after further adjustments were made in the 16th century.

History

Prehistoric calendar

The original Roman calendar is thought to have been based on observing the moon. Each month started with the new moon and lasted about 29 or 30 days. Because the moon’s cycle is shorter than a full year, this calendar would quickly fall out of step with the seasons without adjustments. Some believe early Romans also watched natural signs like tree leaves, bird migrations, and crop growth, along with stars, to keep track of seasons and important farming times.

Legendary 10-month calendar

The Romans told stories about their first calendar having just ten months, from March to December. March, May, July (called Quintilis), and October had 31 days, while the other six months had 30 days. Together these months made up 304 days, leaving winter unorganized. Some tales say this calendar shifted until it was fixed by adding extra days. The Romans often credited their legendary first king, Romulus, with creating this calendar, though historians debate if it truly existed.

Other traditions said Romulus’ calendar was based on the sun and had 360 days, with months added later. Some stories claim another king, Numa, added January and February to make a full year. The Romans also shared an eight-day repeating market cycle with their neighbors, the Etruscans.

Republican calendar

The historical Roman Republic used a twelve-month calendar including January and February for winter. Tradition says King Numa Pompilius added these two months and adjusted the calendar to fit a lunar year of 354 days. To match the solar year, Romans added extra days or a whole extra month in certain years. This kept their calendar aligned with the seasons, though it required careful planning.

The Roman year did not always start in January. Early on, March was the first month, but this changed around 153 BC. Despite changes, the basic structure of adding extra days and months continued until a major reform.

Julian reform

Main article: Julian calendar

Julius Caesar introduced a major change to the Roman calendar in 46 BC. He and his advisors designed a new system that added extra days and set a regular rule for leap years. This Julian calendar brought Roman dates much closer to matching the actual length of the solar year. The year 46 BC was especially long, with extra months added to adjust fully to the new system.

Later reforms

Main article: Byzantine calendar

After Caesar’s death, his follower Mark Antony renamed a month in his honor. Later, Emperor Augustus also renamed a month after himself. The calendar continued with some small changes under Roman emperors, but its basic structure stayed the same for centuries. By the time of the Byzantine Empire, some of these older Roman ways were no longer used in all parts of the empire.

Days

Main article: Kalends

In the Roman calendar, people counted days by looking forward to three important days in each month. These were the Kalends, the first day of the month; the Nones, about a week later; and the Ides, around the middle of the month.

These days were connected to the phases of the moon. The Kalends came after the new moon was first seen, the Nones around the first-quarter moon, and the Ides near the full moon. The Romans used special letters to mark each day on their calendars, showing whether it was a good day for business, meetings, or rest.

Weeks

Main articles: Nundinae, Planetary hours, and Week

The nundinae were special market days in Rome and nearby areas. These days acted like a weekend, happening every eight days. Roman calendars listed each day with a letter from A to H to show where it fit in the market cycle. Each year, the starting letter for markets changed a little.

Later, a seven-day week became common, starting in the early days of the Roman Empire. This came from eastern religions and their traditions. By the time Constantine made Sunday an official day of rest in AD 321, the seven-day week had replaced the older market days.

Months

The names of the months in the Roman calendar were originally used like adjectives before becoming names themselves. Some names are easy to understand: January and March were named after gods Janus and Mars. July and August were named after important leaders Julius Caesar and Augustus. Other months like September, October, November, and December started as words meaning the fifth, sixth, seventh, and eighth months because the calendar began in spring.

In Roman times, people counted days in a special way. They would count forward from special days like the ides or kalends to the next month’s kalends. For example, March 19 was called “the 14th day before the April Kalends.” This way of counting sometimes caused confusion when the calendar changed.

Intercalation

Main article: Mercedonius

The early Roman calendar had 355 days, which was not enough to match the actual length of a year. To fix this problem, the Romans added extra days to the calendar every few years. They did this by adding an extra month called Mercedonius in February. This helped keep the calendar in line with the seasons.

Sometimes people were unsure about when to add these extra days, which caused some confusion with dates. Leaders decided when to add the extra month, and this system worked well for a long time. However, it sometimes broke down due to political issues in Rome.

Years

Main article: List of Roman consuls

The Romans had different ways to name their years. In early times, they often used the names of leaders called consuls to mark the year. Later, during the time of emperors, they used the rulers' years in office. Sometimes, people counted years from the legendary start of Rome, but this was not common.

The Roman year originally started in March, but later changed to begin in January. This shift happened so leaders could start their duties earlier and plan battles better. Different places in the Roman world also had their own ways to keep track of time.

Conversion to Julian or Gregorian dates

People sometimes think that Roman dates match up with our modern calendar dates, but this isn't true. Roman dates can be very different from Julian or Gregorian dates because of changes in how they kept track of time.

Some events from Roman history show just how off the Roman calendar was. For example, in the year we often call 205 BC, the Roman year actually started in March and ended in March of the next year. But because of problems with their calendar, this Roman year might have started as early as November or December of 206 BC in our modern dating system. Even after Rome changed to the Julian calendar, mistakes in adding leap years meant that dates were still a few days off for many years.