Calendar

Around 4000 BC, one of the first calendars was created in Mesopotamia. Mesopotamian astronomers devised a 12-month calendar based on lunar cycles. The year was divided into two seasons: summer and winter. The Babylonian calendar is based on a Sumerian (Third Dynasty of Ur) forefather recorded in the Shulgi Umma calendar (c. 21st century BC). It was a lunisolar calendar, with years made up of 12 lunar months, each commencing when a new crescent moon was first seen low on the western horizon at nightfall, with an intercalary month inserted as needed by decree.

According to Encyclopedia Britannica, the solar year in Mesopotamia was divided into two seasons: "summer," which included the barley harvest in the second part of May or early June, and "winter," which roughly matched today's fall-winter. In northern regions, three seasons (Assyria) and four seasons (Anatolia) were counted, but in Mesopotamia, the division of the year looked natural. Prognoses for the well-being of Mari, on the middle Euphrates, were taken for six months as late as around 1800 BCE.

The months began with the first sighting of the New Moon, and court astronomers still reported this important observation to the Assyrian monarchs in the 8th century BCE. Month names varied from city to city, even within the same Sumerian city of Babylonia, a month could have numerous names derived from festivals, duties (e.g., sheepshearing) typically conducted in the particular month, and so on, according to local demands. On the other hand, as early as the 27th century BCE, the Sumerians utilized artificial time units to refer to the term of a high official—for example, on N-day of PN, the governor's turn of office. The Sumerian government also required a time unit that encompassed the entire agricultural cycle, such as from the delivery of new barley and the settlement of relevant accounts to the following crop.