A calendar is a system for keeping track of days, months, and years. Since ancient times, people have used calendars to record and plan events, such as the planting of crops and the celebration of religious festivals and other special occasions. The term calendar comes from the Latin word kalendae, which refers to the first day of each month in Roman times.

The earliest system for keeping track of time was based on the phases of the moon. In this type of calendar, the lunar calendar, each month corresponds to the time it takes the moon to make one complete cycle through its phases. A year based on lunar months has roughly 354 days. However, a solar year, which is based on the position of the sun from day to day, has 365 days.


The early Romans named several months according to their position in the year. The months of September, October, November, and December originally were the seventh, eighth, ninth, and tenth months. The names of these months came from the Latin words for these numbers— septem, octo, novem, and decem. Later reforms changed the position of these months in the year, but the names stayed the same. The Romans renamed the fifth (quintilis) and sixth (sextills) months July and August, after Julius Caesar and Augustus Caesar—the rulers who helped perfect the calendar. The remaining months were named after gods and festivals.

Over a period of time, the difference between the length of the lunar year and the length of the solar year creates problems. For example, it is possible for a spring month on the lunar calendar to fall in the middle of summer on the solar calendar. This problem troubled ancient peoples because many of their festivals were related to the seasons and had to be celebrated at a particular time. They believed that they risked angering the gods if they celebrated at the wrong time. The Greeks and Romans found ways to resolve this problem.

Greek Calendars. The early Greeks used a lunar calendar with 12 months, some with 29 days and some with 30. Each city-state had its own names for the months and the days of the month, as well as its own sequence of months. The Greeks organized the days into groups of ten called decades, but the last decade of 29-day months had only nine days. (The English word decade refers to a period of ten years rather than ten days.) The Greeks often kept track of years by naming them, usually after important people (such as rulers or priestesses) or events.

At first, the Greeks made no adjustments to resolve the difference between the lunar and solar year. When they did make changes, by adding extra days or a month every few years, each city followed its own system. As a result, no standard calendar existed in ancient Greece. During the 500s and 400s B.C., Greek astronomers devised more precise ways of adjusting the lunar year to match the solar year. They created various cycles in which a set number of days would be added over a certain period of years. Over time, these “astronomical cycles” became more precise and helped bring greater accuracy to calendars.

Roman Calendars. The early Romans also used a lunar calendar with months that ranged from 29 to 31 days. Unlike the Greek calendars, however, the early Roman calendars consisted of only ten months and contained 295 days. Martius (March) was the first month of the year. In about 700 B.C., the Romans added two more months to their calendar, bringing it more into line with the solar year.

The months of the Roman calendar were organized into groups, and certain days had special names. In addition to the kalends (kalendae), the first day of the month, the Romans had the nones (nonae), the 5th or 7th day of the month, and the ides (idus), the 13th or 15th day. The Ides of March (March 15) became famous as the day Julius Caesar was assassinated. Like the Greeks, the Romans also kept track of years by naming them after people or events.

For centuries, the Romans tried to make adjustments to their calendar to resolve the difference between the lunar and solar year. By 46 B.C. their calendar no longer corresponded to the seasons. For this reason, Julius Caesar launched a series of calendar reforms in an attempt to resolve the problem and standardize the calendar. First, he lengthened the year 46 B.C. to 445 days to bring the calendar in line with the seasons. Then, he fixed the length of a year at 365 days, with one extra day added in February every four years. He also revised the sequence of months so that the year started with Januarius (January) and established a pattern for months with 29, 30, and 31 days. This new Julian calendar, named in honor of Caesar, was far more accurate than the earlier ones.

Unfortunately, Roman officials did not fully understand the rules about adding days to the Julian calendar. Instead of adding an extra day every four years, they added one every three years. As a result, the calendar once again fell out of sync with the seasons. The emperor Augustus corrected this problem during his reign and established the proper cycle.

As Roman civilization spread, other cultures adopted the Julian calendar, and it became the standard throughout the Roman Empire. Further adjustments were made in the A.D. 300s. The Gregorian calendar, the one in use today, was adopted in the A.D. 1500s. It differs only slightly from the Julian calendar.

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