The beginning of the Babylonian year starts at the Spring Equinox. (Note 1) This turning of the year is called zagmu, “the border of the year.” At this time, people take stock, review their personal affairs, and check their financial accounts. Then they affirm the Gods as the supreme authorities of the cosmos.
According to Assyrian Astrolabe B, Nisannu, the first month of the year is for Nanna (Sin), the God of the Moon. “The month Nisannu, the dais of Anu, the king is installed and invested (with authority); the month of Nanna-Suen, the first-born of Enlil.” An incantation from Nimrud reiterates this: “May the month Nisannu, (the month) of Anu and Enlil, absolve! The first month belonged to the First of the Gods.”
During Nisannu (the new moon after the equinox) the Akitu, the New Year Festival is held for twelve days. It starts with purifications, and then the Enuma Elish (the Babylonian Creation Epic) is read. This myth begins with the original creation of the world by Tiamat, the Goddess of Chaos, and Apsu, the God of Waters. Later Anu, a God from the succeeding generation becomes the “Father of the Gods.” Eventually, He cedes his powers to Enlil, from yet a newer generation of Gods, who seeks to overthrow the original Gods. After Enlil kills Apsu, Tiamat wages war on the newer Gods. In desperation, Enlil goes to Marduk, the principal deity of Babylon, for help. On the condition that He is made the Ruler of the Gods, Marduk agrees. After killing Tiamat, Marduk remakes the world from her body.
During the Akitu, Marduk disappears. While his and Nabu’s temples are being cleansed, the people search for Him. At this time, they carry the statues of the other Gods to Marduk’s temple. Meanwhile, Nabu, the Scribe of the Gods and Marduk’s Minister, searches for and then frees Marduk from the Underworld. Then in his temple, the priests re-enthroned Marduk as the Ruler of the Gods. (Note 2) Afterwards, they do divination for the coming year. The festival ends with celebrations and the return of the Gods to their shrines.
Meanwhile to begin the growing season, the King would enact a sacred marriage with the temple priestess of Ishtar (Inanna). Their mating is to reaffirm the marriage of Ishtar, the Goddess of Fertility, with her husband, Tammuz (Dumuzi). These marriage rites ensure that the King is accepted as one of the Gods, and blessed by Ishtar, who also blesses the crops.
Note 1: The Babylonians had a lunar calendar, and added months beyond the 12-month year. They kept their year keyed to the equinoxes. Between 1750 – 1500 BCE, they standardized their calendar – the Standard Mesopotamian Calendar to unify their empire.
Note 2: This called the “Installation of the King,” which could either mean Marduk, Himself or a human king.