Nisanu was the first month of the ancient Mesopotamian lunar calendar in Akkadian usage.
Nisanu began in spring and generally corresponded to March or April in the modern Gregorian calendar, although the exact timing varied because the calendar was based on lunar observations. The month was associated with the Babylonian New Year festival, Akitu, which celebrated the renewal of kingship and cosmic order.
The name Nisanu is Akkadian. In the related Hebrew calendar, the corresponding month is called Nisan, though the calendars and religious traditions are not identical. Mesopotamian calendars also used intercalary months when necessary to keep lunar months aligned with the agricultural and seasonal year.
Nisanu should not be confused with Tashritu, the seventh month, which also had major festivals. Month names could vary by language and region, but Nisanu was conventionally listed first in the Babylonian calendar.