A calendar is a tool used to mark the passing of time by dividing it into years, months, weeks, and days. People of ancient times based the earliest calendars on the most obvious regular events they knew—the changing positions of the sun, moon, and stars. These calendars helped them figure out when to plant and harvest their crops. Over time different groups of people developed other calendars based on their own needs and beliefs.

Days, weeks, months, and years

Several units of time are common to virtually all calendars. The day is the most basic unit. It is based on the length of time the Earth takes to rotate once on its axis. A group of seven days is called a week. Unlike the day, the week is not based on any astronomical event. The month is based on the amount of time needed for the moon to complete a full cycle of its phases—about 29¹/₂ days. The year is the time taken by the Earth to travel around the sun—about 365 days.

These lengths of time do not fit together easily. Twelve cycles of the moon, for example, take 354 days, not 365. The difficulty comes from the fact that the month is lunar, or based on the moon, but the year is solar, or based on the sun. The seasons are tied to the sun and therefore match up with the solar year. To make the lunar months fit into the solar year—and therefore agree with the seasons—it is necessary to add days at certain times. This is why calendar months differ in length. All except February are longer than 29 days so that together they make up for the extra 11 days of the solar year. For the same reason an additional day is added to February every fourth year. These are called “leap years.”

Early calendars

The Sumerians of ancient Mesopotamia (now Iraq) used the moon's phases to come up with a calendar with 12 months in a year. To make this lunar calendar match the seasons, they added an extra month about every four years. This calendar served as a model for others, including the Jewish calendar.

The early Egyptians also used a calendar based on the moon. Later, though, the Egyptians worked out a calendar that corresponded almost exactly to the seasons. This calendar was the first in which a year had 365 days. The Egyptians divided the year into 12 months of 30 days each, with five extra days at the end.

A variety of calendars were used among the different Greek peoples. Broadly, the Greek calendar was similar to the Sumerian calendar, with 354 days divided into 12 lunar months. To try to keep in step with the solar year, the Greeks added an extra month every other year. But the resulting 737 days was seven days longer than two solar years, so the Greek calendar was still wrong. The Greeks in some cities ended up adding or dropping months and days at will to try to keep up with the solar year.

The calendar developed in ancient China was basically lunar. Its year consisted of 12 months of alternately 29 and 30 days. Again, months were sometimes inserted to keep the calendar year in step with the solar year. Traditionally, the Chinese gave their years one of a series of 12 animal names. These names in order are rat, ox, tiger, hare, dragon, snake, horse, sheep, monkey, fowl, dog, and pig.

The Hindu calendar, in use in India from about 1000 BC, was based on the moon. It had 354 days divided into 12 months. To keep in step with the solar year an extra month was added every 30 months, but the resulting calendar was still incorrect.

The early Romans had a lunar year of 355 days. The calendar was regulated by the high priest. On the calends, or day of the new moon, he announced the times of the nones (first quarter of the moon) and ides (full moon) for that month. To make the lunar year agree with the solar year, priests and politicians added extra months whenever they wanted to. As a result the calendar soon became a complete muddle.

The Julian calendar

The Roman leader Julius Caesar made important corrections to the calendar. In 46 BC he decided on the figure of 365 ¹/₄ days for the length of the solar year. To account for this figure in the calendar, he ordered that the years should have 365 days each, except every fourth year. To use up the quarter of a day left over from each ordinary year on the calendar, every fourth year had 366 days and became known as “leap year.” This calendar, named “Julian” after its creator, has the 12 months at the lengths used today.

The Julian calendar was used for roughly the next 1,600 years. During this time, however, a problem arose. The Julian year was about 11 minutes longer than the year of the seasons. Although 11 minutes a year is not much in itself, the number of minutes gradually grew into days as the centuries passed.

The Gregorian calendar

By the 16th century the error in the Julian calendar amounted to ten days. Easter and other festivals of the Roman Catholic church were so out of place that Pope Gregory XIII ordered ten days to be dropped from the year 1582. To keep the calendar correct in the future, Gregory ordered that leap year should be skipped three times in every 400 years. This system is called the Gregorian, or New Style, calendar. The Julian calendar became known as the Old Style calendar.

The names for the months in the Gregorian calendar were taken from the ancient Roman months of the Julian calendar. January comes from Janus, the household god. February was the time of a feast called Februa. March was named for Mars, god of war. April may have been named for Aphrodite, the Greek goddess of love and beauty. May probably comes from the goddess Maia. June was named for the goddess Juno. July and August were named after Julius Caesar and his son, Augustus. The last four months got their names from numbers. “September,” for example, comes from the Latin word for “seven,” “October” from “eight,” and so on. These names indicated the placement of the months in the Roman calendar.

Despite the improvements made by the Gregorian calendar, it was not immediately accepted everywhere. Most Roman Catholic states adopted the new system by 1587. Some Protestant states embraced it around the beginning of the 18th century, but a number of others, such as Great Britain and its colonies, did not do so until the 1750s. Japan, China, and Russia, to name only a few, adopted the Gregorian rules much later. Today the Gregorian calendar is used throughout much of the world.

Native American calendars

The Maya and Aztec had the most advanced calendars among the Native American peoples who lived in the Americas before Europeans arrived. The basic structure of the Mayan calendar is common to all calendars of ancient Mexico and Central America. It consisted of a ritual cycle of 260 named days and a year of 365 days. The year was divided into 18 months of 20 days each, with five days added to fill out the years. These last five days were considered to be unlucky. The calendar of the Aztec also had a year of 365 days broken down into 18 months of 20 days, plus the five extra days to complete the year. But the Aztec and Mayan years did not necessarily coincide with one another.

Other calendars

Several calendars other than the Gregorian calendar are still in use. The Muslim calendar, for example, is still used by most Arab countries. It is a lunar calendar with 12 months of, alternately, 30 and 29 days, making a year of 354 or 355 days. No months are added to make the calendar match up with the solar year, which means that the named months have no relation to the seasons. As a result, major festivals may occur in any season. The Chinese calendar is still used along with the Gregorian calendar in China, Taiwan, and neighboring countries. The traditional Hindu and Jewish calendars continue to be used for religious purposes.