A calendar is a system of organizing days. This is done by giving names to periods of time, typically days, weeks, months and years. A date is the designation of a single, specific day within such a system.
A calendar is convenient for regulating civil life and religious observances and for historical and scientific purposes.
The word “calendar” is derived from the Latin calendarium, meaning “interest register” or “account book,” itself a derivation from calendae (or kalendae), the first day of the month in the Roman republican calendar, the day on which future market days, feasts, and other occasions were proclaimed.
The year measured by the Earth’s orbit around the sun is roughly an unruly 365.2422 days. The moon is likewise not a fan of whole numbers. In the space of a year, there are around 12.3683 lunar months. Societies have traditionally tried to make sure that the same seasons lined up with the same months.
The Sumerians were the first to use calendars in Mesopotamia during the Bronze Age. Each month on this calendar had 29 or 30 days, depending on whether or not the first day had a full moon. Regardless, there were always 12 months in a year.
The Sumerian calendar was followed by the Egyptian, Assyrian and Elamite calendars.
The Vedic India developed a sophisticated time keeping methodology and calendars for Vedic rituals.
A large number of Ancient Near East calendar systems based on the Babylonian calendar date from the Iron Age, among them the calendar system of the Persian Empire, which in turn gave rise to the Zoroastrian calendar and the Hebrew calendar.
A mesolithic arrangement of twelve pits and an arc found in Warren Field, Aberdeenshire, Scotland, dated to roughly 10,000 years ago, has been described as a lunar calendar and was dubbed the “world’s oldest known calendar” in 2013.
The Oldest European calendar other than the one found in Scotland, is the one found near to Vukovar in modern-day Croatia. It is a ceramic vessel bearing inscribed ideograms of celestial objects.
The Greeks had many lunisolar calendars they used to keep track of time. The Athenian calendar, also known as the attic or civil calendar, was the most common, but the Greeks also created the Olympiad, Seasonal, Conciliar, and Metonic calendars. Each one was based on the cycle of the moon and stars as well as solar equinoxes.
Calendars in antiquity were usually lunisolar, depending on the introduction of intercalary months to align the solar and the lunar years. This was mostly based on observation, but there may have been early attempts to model the pattern of intercalation algorithmically, as evidenced in the fragmentary 2nd-century Coligny calendar.
The first Roman calendar was introduced by King Romulus. This calendar had only 10 months, starting in March and ending in December. A lunar year had 354 days, but since the Romans believed even numbers were lucky, they changed things around so that each month had an even number of days. This caused the seasons to be out of sync year after year!
The Julian calendar, named after Julius Caesar’s reforms of 46/45 BC, approximated the solar year to 365.25 days and inserted an extra day each four years. That left a rather annoying 11 and a bit minutes unaccounted for. More on those minutes later.
In the 11th century in Persia, a calendar reform led by Khayyam was announced in 1079, when the length of the year was measured as 365.24219858156 days. Given that the length of the year is changing in the sixth decimal place over a person’s lifetime, this is outstandingly accurate. For comparison the length of the year at the end of the 19th century was 365.242196 days, while today (1999) it is 365.242190 days.
The Gregorian calendar is the calendar used in most of the world. It was introduced in October 1582 by Pope Gregory XIII as a modification of, and replacement for, the Julian calendar. The principal change was to space leap years differently so as to make the average calendar year 365.2425 days long, more closely approximating the 365.2422-day ‘tropical’ or ‘solar’ year that is determined by the Earth’s revolution around the Sun.
In 1752, the United States adopted the Gregorian calendar. At this point, 10 countries were already using this system including Italy, France, Germany, Portugal, and Switzerland.
The Hebrew calendar was created in Israel. This calendar is lunisolar and is heavily based on mathematics. Today, the Hebrew calendar is still used for Jewish religious observations.
Many people think of “Doom’s Day” (December 21, 2012) when they think of the Mayan calendar, and there was even made a movie made about it. According to the BBC News, however, this was “a big understanding.” The calendar ended in 2012 simply because it was the end of one cycle and the beginning of another.