The Babylonian calendar was a lunisolar calendar used in Mesopotamia from around the 2nd millennium BC until the Seleucid Era (294 BC), and it was specifically used in Babylon from the Old Babylonian Period (1780s BC) 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.^[1] The lunisolar calendar descends from an older Sumerian calendar used in the 4th and 3rd millennium BC.^[2]

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.^[3]

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

The Babylonian civil calendar, also called the cultic calendar, was a lunisolar calendar descended from the Nippur calendar, which has evidence of use as early as 2600 BCE and descended from the even older Third Dynasty of Ur (Ur III) calendar. The original Sumerian names of the months are seen in the orthography for the next couple millennia, albeit in more and more shortened forms. When the calendar came into use in Babylon circa 1780 BCE, the spoken month names became a mix from the calendars of the local subjugated cities, which were Akkadian. Historians agree that it was probably Samsu-iluna who effected this change.^[3] During the sixth century BCE Babylonian captivity of the Jews, these month names were adopted into the Hebrew calendar.

The first month of the civil calendar during the Ur III and Old Babylonian periods was Šekinku (Akk. Addaru), or the month of barley harvesting, and it aligned with the vernal equinox. However, during the intervening Nippur period, it was the twelfth month instead.^[3]

Until the 5th century BCE, the calendar was fully observational, and the intercalary month was inserted approximately every two to three years, at first by guidelines which survive in the MUL.APIN tablet. Beginning in around 499 BCE, the intercalation began to be regulated by a predictable lunisolar cycle, so that 19 years comprised 235 months.^[3] Although this 19-year cycle is usually called the Metonic cycle after Meton of Athens (432 BCE), the Babylonians used this cycle before Meton, and it may be that Meton learned of the cycle from the Babylonians.^[4] After no more than three isolated exceptions, by 380 BCE the months of the calendar were regulated by the cycle without exception. In the cycle of 19 years, the month Addaru 2 was intercalated, except in the year that was number 17 in the cycle, when the month Ulūlu 2 was inserted instead.^[3]

During this period, the first day of each month (beginning at sunset) continued to be the day when a new crescent moon was first sighted—the calendar never used a specified number of days in any month. However, as astronomical science grew in Babylon, the appearance of the new moon was predictable with some accuracy into the short-term future. Still, during the Neo-Assyrian period (c. 700 BCE) the calendar was sometimes retroactively "shifted back" a day to account for the fact that the king should have declared a new month, but only did so the following day because of obstructive weather.^[5]

As a lunisolar calendar, the civil calendar aimed to keep calendar months in sync with the synodic month and calendar years in sync with the tropical year. Since new months of the civil calendar were declared by observing the crescent moon, the calendar months could not drift from the synodic month. On the other hand, since the length of a calendar year was handled by the Metonic cycle starting after 499 BCE, there is some inherent drift present in the formulaic computation of the new year when compared to the true new year. While on any given year the first day of the first month could be up to 20 days off from the vernal equinox, on average the length of a year was very well approximated by the Metonic cycle; the computed average length is within 30 minutes of the true solar year length.^[12]

Since the civil calendar was not standardized and predictable for at least the first millennium of its use, a second calendar system thrived in Babylon during the same time spans, known today as the administrative or schematic calendar. The administrative year consisted of 12 months of exactly 30 days each. In the 4th and 3rd millennia BCE, extra months were occasionally intercalated (in which case the year is 390 days), but by the beginning of the 2nd millenium BCE it did not make any intercalations or modifications to the 360-day year.^[3] This calendar saw use in areas requiring precision in dates or long-term planning; there is tablet evidence demonstrating it was used to date business transactions and astronomical observations, and that mathematics problems, wage calculations, and tax calculations all assumed the administrative calendar instead of the civil calendar.^[1]

Babylonian astronomers in particular made all astral calculations and predictions in terms of the administrative calendar. Discrepancies were accounted for in different ways according to the heavenly measurements being taken. When predicting the phase of the moon, it was treated as if each ideal month began with a new moon, even though this could not be true. In fact, this guideline appears in the MUL.APIN, which goes on further to specify that months that began "too early" (on the 30th of the previous month) were considered unlucky, and months that began "on time" (the day after the 30th of the previous month) were considered auspicious. When discussing the dates of equinoxes and solstices, the events were assigned fixed days of the administrative calendar, with shortening or lengthening of intervening days taking place to ensure that the celestial phenomena would fall on the "correct" day. Which fixed day each phenomenon was assigned varied throughout time, for one because which month was designated first varied throughout history. In general, they were assigned to the 15th day of four equally spaced months.^[3]

Counting from the new moon, the Babylonians celebrated every seventh day as a "holy-day", also called an "evil-day" (meaning "unsuitable" for prohibited activities). On these days officials were prohibited from various activities and common men were forbidden to "make a wish", and at least the 28th was known as a "rest-day". On each of them, offerings were made to a different god and goddess, apparently at nightfall to avoid the prohibitions: Marduk and Ishtar on the 7th, Ninlil and Nergal on the 14th, Sin and Shamash on the 21st, and Enki and Mah on the 28th. Tablets from the sixth-century BC reigns of Cyrus the Great and Cambyses II indicate these dates were sometimes approximate. The lunation of 29 or 30 days basically contained three seven-day weeks, and a final week of eight or nine days inclusive, breaking the continuous seven-day cycle.^[13]

Among other theories of Shabbat origin, the Universal Jewish Encyclopedia of Isaac Landman advanced a theory of Assyriologists like Friedrich Delitzsch^[14] that Shabbat originally arose from the lunar cycle,^[15][16] containing four weeks ending in Sabbath, plus one or two additional unreckoned days per month.^[17] The difficulties of this theory include reconciling the differences between an unbroken week and a lunar week, and explaining the absence of texts naming the lunar week as Shabbat in any language.^[18]

The rarely attested Sapattu^m or Sabattu^m as the full moon is cognate or merged with Hebrew Shabbat, but is monthly rather than weekly; it is regarded as a form of Sumerian sa-bat ("mid-rest"), attested in Akkadian as um nuh libbi ("day of mid-repose"). According to Marcello Craveri, Sabbath "was almost certainly derived from the Babylonian Shabattu, the festival of the full moon, but, all trace of any such origin having been lost, the Hebrews ascribed it to Biblical legend."^[19] This conclusion is a contextual restoration of the damaged Enûma Eliš creation account, which is read as: "[Sa]bbath shalt thou then encounter, mid[month]ly."^[13]

The Akkadian names for months surface in a number of calendars still used today. In Iraq and the Levant, the solar Gregorian calendar system is used, with Classical Arabic names replacing the Roman ones,^[20] and the month names in the Assyrian calendar descend directly from Aramaic, which descended from Akkadian.^[21] Similarly, while Turkey uses the Gregorian calendar in the present day, the names of Turkish months were inspired by the 1839 Rumi calendar of the Ottoman Empire, itself derived from the Ottoman fiscal calendar of 1677 based on the Julian calendar. This last calendar month names of both Syriac and Islamic origin, and in the modern calendar four of these names descend from the original Akkadian names.^[22]

• Parker, Richard Anthony and Waldo H. Dubberstein. Babylonian Chronology 626 BC.–AD. 75. Providence, RI: Brown University Press, 1956.
• W. Muss-Arnolt, The Names of the Assyro-Babylonian Months and Their Regents, Journal of Biblical Literature (1892).
• Sacha Stern, "The Babylonian Calendar at Elephantine" in Zeitschrift für Papyrologie und Epigraphik 130 (2000) 159–171 (PDF document, 94 KB)
• Fales, Frederick Mario, “A List of Umma Month Names”, Revue d’assyriologie et d’archéologie orientale, 76 (1982), 70–71.
• Gomi, Tohru, “On the Position of the Month iti-ezem-dAmar-dSin in the Neo-Sumerian Umma Calendar”, Zeitschrift für Assyriologie und Vorderasiatische Archäologie, 75 (1985), 4–6.
• Pomponio, Francesco, “The Reichskalender of Ur III in the Umma Texts”, Zeitschrift für Assyriologie und Vorderasiastische Archäologie, 79 (1989), 10–13.
• Verderame, Lorenzo, “Le calendrier et le compte du temps dans la pensée mythique suméro-akkadienne”, De Kêmi à Birit Nâri, Revue Internationale de l'Orient Ancien, 3 (2008), 121–134.
• Steele, John M., ed., "Calendars and Years: Astronomy and Time in the Ancient Near East", Oxford: Oxbow, 2007.

• The Babylonian Ritual Calendar
• The Mul.Apin Tablets
• Structure of the Babylonian calendar
• The Babylonian Calendar (with a date converter based on Parker & Dubberstein (1971))