Babylonian calendar

The Babylonian calendar was a lunisolar calendar used in Mesopotamia from around the second millennium BCE until the Seleucid Era (294 BCE), and it was specifically used in Babylon from the Old Babylonian Period (1780 BCE) 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.

The civil lunisolar calendar had years consisting of 12 lunar months, each beginning when a new crescent moon was first sighted low on the western horizon at sunset, plus an intercalary month inserted as needed, at first by decree and then later systematically according to what is now known as the Metonic cycle.

Month names from the Babylonian calendar appear in the Hebrew calendar, Assyrian calendar, Syriac calendar, Old Persian calendar, and Turkish calendar.