Calendar comes from Romans calling out the new moon
A calendar names days, weeks, months and years so people can agree on dates. Most track the sun, the moon or both. The English word comes from Latin kalendae, the first day of the Roman month, linked to calare, to call out, for announcing the first sighting of the new moon.
Roman debts were settled on the calends, so calendarium came to mean an account book; the word passed through Old French calendrier into Middle English by the 13th century. The earliest written calendars came from Bronze Age Egypt and Sumer. Egyptians built a 365-day year of twelve 30-day months plus five extra days, anchored to the dawn return of Sirius that coincided with the Nile flood, but ignored the leftover fraction of a day, so their year slowly slipped. Babylonian reckoning shaped the Persian, Zoroastrian and Hebrew calendars, while Vedic India developed its own ritual calendar independently.
Ancient calendars were mostly lunisolar, inserting an extra month when needed to keep lunar months in step with the seasons; China's calendar and the Hebrew calendar with its 19-year cycle still work this way. Julius Caesar broke Rome's link to the moon in 46 BC with a leap day every four years, and the Gregorian reform of 1582 fixed most of the remaining drift. The Islamic calendar went the other way: Muhammad's ban on intercalation, traditionally dated to 632, left a purely lunar year that wanders through the seasons. Rome's earliest calendar did not even name its 61 winter days, which later became January and February.
Designers face a trade-off. Observation-based calendars, such as the religious Islamic one, are always accurate but make future dates hard to predict; rule-based ones, such as today's Jewish calendar, are easy to compute but drift, partly because Earth's rotation changes. The seven-day week follows no astronomical cycle yet has run unbroken for millennia. The simplest systems just count days from a starting point, as Julian day numbers and Unix time do.
Some countries counted years by reigns: 2006 in Japan was year 18 of Heisei, Emperor Akihito's era. Reform plans such as the World Calendar and the International Fixed Calendar have stalled because switching would disrupt continuity and religious cycles. Alexander Marshack argued, controversially, that notches on a bone from about 25,000 BC tracked the moon.
Source: Calendar
