The ancient Babylonian New Year festival Akitu traditionally lasted 12 days.
Akitu was celebrated in the first month of the Babylonian year, Nisanu, during the spring rather than on January 1. The festival honored the chief god Marduk at Babylon and involved processions, ceremonies, prayers, and the participation of the king. Its rituals helped renew the relationship between the city, its ruler, and the gods.
A major part of the festival involved carrying divine images in a procession between Babylon and a special Akitu house outside the city. The Babylonian creation epic Enuma Elish was recited, reinforcing Marduk's cosmic authority. The king also took part in a ritual of humiliation and restoration before the high priest, symbolizing the renewal of legitimate rule.
Akitu is often mentioned in discussions of the earliest known New Year celebrations, but it was not the direct source of today's January 1 calendar date. Babylonian calendars were lunisolar, and the festival's timing followed the spring month Nisanu.