Nisan in the Babylonian and Hebrew calendars is the month of the barley ripening and first month of spring. The name of the month is an Akkadian language borrowing, although it ultimately originates in Sumerian nisag "first fruits". In the Hebrew calendar it is the first month of the ecclesiastical year, called the "first of the months of the year", "first month", and the month of Aviv בְּחֹ֖דֶשׁ הָאָבִֽיב ḥōḏeš hāʾāḇîḇ). It is called Nissān in the Book of Esther. It is a month of 30 days. In the year 2024, 1 Nisan will occur on 9 April. Counting from 1 Tishrei, the civil new year, it would be the seventh month, but in contemporary Jewish culture, both months are viewed as the first and seventh simultaneously, and are referred to as one or the other depending on the specific religious aspects being discussed.

The Babylonian calendar was a lunisolar calendar used in Mesopotamia from around the second millennium BCE until the Seleucid Era, and it was specifically used in Babylon from the Old Babylonian Period until the Seleucid Era. The civil lunisolar calendar was used contemporaneously with an administrative calendar of 360 days, with the latter used only in fiscal or astronomical contexts. The lunisolar calendar descends from an older Sumerian calendar used in the 4th and 3rd millennia BCE.