Roman calendar

The Roman calendar was used by the Roman Kingdom and Roman Republic. It started with a simple system made by Romulus, Rome's first king. This early calendar had ten months, from March to October, and did not include the winter days. Each month had 30 or 31 days and was split into eight-day cycles called nundinal cycles, which ended with markets and festivals.

Later, Numa Pompilius, who came after Romulus, added January and February to cover the winter months. He also tried to match the calendar to the sun's path by adding extra days, a process called intercalation. But it was hard to keep the calendar right, and over time it lost track of the seasons.

Finally, Julius Caesar changed the calendar in 46 BC. He made the Julian calendar, which followed the sun's path and added a leap day every four years to stay in balance. This system was used for many years and later became the basis for the calendar we use today, the Gregorian calendar, after more changes were made in the 16th century.

History

Prehistoric calendar

The first Roman calendar was based on the moon. Each month began 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 lose track of the seasons. Some think early Romans also watched nature, like tree leaves, bird movements, and crop growth, to follow the 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. The other six months had 30 days. Together these months made 304 days, leaving winter without months. Some stories say this calendar changed until it was fixed by adding extra days. The Romans often said their legendary first king, Romulus, made this calendar, though historians debate if it really existed.

Other traditions said Romulus’ calendar was based on the sun and had 360 days, with months added later. Some tales say 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 changed 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 some years. This kept their calendar aligned with the seasons, though it needed 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 idea of adding extra days and months continued until a big change happened.

Julian reform

Main article: Julian calendar

Julius Caesar made a big change to the Roman calendar in 46 BC. He and his advisors made a new system that added extra days and set a regular rule for leap years. This Julian calendar brought Roman dates much closer to the real length of the solar year. The year 46 BC was very 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 kept changing a little 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 everywhere.

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 linked to 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.

Weeks

Main articles: Nundinae, Planetary hours, and Week

The nundinae were special market days in Rome and nearby places. These days were like a weekend, happening every eight days. Roman calendars used letters from A to H to show where each day 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 started as words used like adjectives before they became month names. 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, but a year is longer than that. To fix this, the Romans added extra days every few years. They added an extra month called Mercedonius in February. This helped keep the calendar matching the seasons.

Sometimes people did not know when to add these extra days, which caused confusion. Leaders decided when to add the extra month, and this worked well for a long time.

Years

Main article: List of Roman consuls

The Romans had different ways to name their years. In early times, they used the names of leaders called consuls to mark the year. Later, during the time of emperors, they used the rulers' years in office.

The Roman year originally started in March, but later changed to begin in January. 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 Roman dates match 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 different 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.