The most prominent and significant observance in the Mesopotamian calendar is that of 𒀉𒆠𒋾 akitu, a multi-day festival which takes place twice a year, at the start of the first and seventh months. Each akitu respectively represents the (Northern Hemisphere) vernal and autumnal equinox.
Although often translated as a "new year" festival, Sumerians actually symbolically divided the year into two equal halves at each equinox, with the sun and moon - and their respective gods, Nanna and Utu - contending with each other for influence and power in the skies in the intervening time.
This division also draws significance from the Mesopotamian agricultural calendar - the vernal equinox heralded hot days and the strenuous labour of the harvest to come, while the autumnal equinox showed that the land was ready to be replenished and reseeded after the scorching heat of summer.
The Sumerians believed that the moon ruled over the sun; Nanna is the father of Utu, and it is by the waxing and waning of the moon that they measured their months and festivals. The Sumerian king Išme-Dagan addresses Nanna, in a praise poem, as:
"Father Nanna, who fixes the months and the new moon, who establishes the year[...]"
It is in Nanna's name that the first attested akitu festivals were celebrated, with records going back to the pre-Sargonic era in the ancient city of Ur. At the time, Nanna was the chief deity of Ur, and the akitu a recreation of his triumphal entrance into that city. This was particularly the case for the akitu of the seventh month, corresponding to the autumnal equinox, after which the moon would be seen for longer in the night sky.
Because of the resemblance of the waxing crescent moon to a boat, Nanna was held to enter into Ur on a barge known as the Boat of Heaven. The original akitu was a recreation of this entrance; on the first day of the akitu, Nanna's statue would be taken in an elaborate procession to the akitu-house, which stood outside the city of Ur and was only accessible by barge. There, he would receive the normal ritual offerings due to him, until the akitu culminated in his triumphal return to the city amid a similarly - if not even more - rich and colourful procession.
The akitu-house fulfilled little role by itself; although offerings would be made to Nanna there, the festivities still took place within the city. The akitu-house represents the primordial dwelling of the gods, and the return procession the act of celebrating the moment when that deity took up residence in their city.
At some time in the 3rd millennium BCE, the akitu was adopted by the city of Nippur in its function as a Sumerian religious centre. It was adopted to the religious calendar of Nippur and its chief god, Enlil, losing some of the symbolism that it had had to the worship of Nanna at Ur. From there, it was adopted by other cities as a way to honour each city's patron deity, and to recreate and celebrate that deity's entrance to their city. For showing due respect and reverence, that city's chief deity would decree a good fate for their city and ensure the administration of justice within its walls.
Although it was a multi-day festival, the length and timing of the akitu varied between cities. In Ur the akitu of the seventh month, representing the triumph of the moon over the sun, was eleven days, compared to five days in the first month. One akitu lasted seven days in Aššur, and another lasted twelve days in Babylon, where - on the fifth day - the king would famously be struck by the High Priest of Marduk, denoting the subservience of the kingly office to the gods.
The akitu, in ancient times, was carried out as part of an elaborate state and civic religion, using resources that are not necessarily available to the modern practitioner. Furthermore, the content and details of each city's akitu would vary based on local needs and the nature of the deity being honoured.
Because of this, it is impossible to detail the specifics of what an akitu might look like to each modern practitioner. However, The Ishtar Gate provides this outline for a seven-day akitu adapted from those observed at Ur, Nippur, Babylon and elsewhere which can be further adapted to each individual's need.
In the following text, "year" can be freely substituted for "equinox-year" or "half-year".
The day before the akitu, the last day of the preceding month, should be spent in sombre reflection for the events of the preceding year. It is a time to express repentance for the wrongdoing of the past year, but also to give thanks for the blessings we have received and the personal growth that we have made in the same time.
Day 1: may first involve feasting for the onset of the new year, but the principal focus of this day should see any icons or statues of deities taken from their dwelling places and altars, and carried reverently to another place, preferably outside the room or even (if possible) the home they normally reside in. This new dwelling place is the ceremonial akitu-house, and the gods enshrined in their temporary dwelling with offerings and prayers.
Day 2: a time of lamentation and sorrow for the absence of the deity or deities from their habitual residence. Throughout the period of akitu, the gods should receive their normal offerings in their ceremonial akitu-house, and prayers and compositions to the deities being honoured should be recited; on this day, lamentation hymns are an appropriate choice. For the modern practitioner, a day to reflect on how and why the gods came into your life, and your journey since then.
Day 3: we reflect on the meaning of the new year as a time of creation and renewal, just as matter was created from the primordial ocean. Prayers and offerings of thanks; a creation story to be recited, such as the Babylonian Enuma Eliš or the prologue to the Sumerian version of the Gilgameš epic.
Day 4: a day of atonement, a day on which we allow our hearts to be open and vulnerable before our gods. We should be sincere about our transgressions, our sorrows, and our intentions for the coming year. What this looks like is deeply personal to the individual, but sincerity and outpouring of emotion are valued above all else.
Day 5: preparation of the altar space or room for the re-entry of the gods. Cleaning and purification. In doing so, we cleanse ourselves, our homes, and the dwelling spaces of the gods of the emotional and spiritual impurities of the previous year. Recite a text or composition with personal, hopeful meaning.
Day 6: the gods assemble to determine the fates. Offerings and praise to be given in the hope of good fate for the year ahead; an ideal offering is a personal creation or composition that is to be dedicated to one's primary patron. Reflections on fate, such as Enki and the World Order, are good to recite on this day.
Day 7: the gods should be taken in triumphant procession back to their regular dwelling place, and enshrined in their appropriate place of honour, with due ceremony and joyful prayer. Normal administration of their dwelling place can resume, typically with the observation of the first-quarter moon that customarily falls on the 7th of the month.