Solar and lunar calendars

The Hebrew Bible is littered with dates of events and timescales in one form or another. When these are examined it is apparent that separate solar calendars and luni-solar calendars are used in different parts of the Bible. Luni-solar calendars are those in which the months and years are defined by the moon, with 30-day leap months intercalated, that is, inserted into the calendar, to keep the lunar calendar in line with the sun’s year and the seasons. There is little agreement among modern scholars about the nature and structure of these calendars.

Genesis 1:3-2:3 describes the seven days of Creation with each day given an ordinal number from the first to the seventh. The Sabbath is the seventh day. The Bible does not specify days of the week although the days of the month may be mentioned.

The Mesopotamian calendar

The lunar months and the solar months are both numbered from the first to the twelfth. Although intercalary months are not listed as the thirteenth, that does not mean that they are not included. The Mesopotamian calendar upon which it is presumed the luni-solar Israelite calendar was based, intercalated a 30-day month seven times in 19 solar years at two to three-year intervals. This included a second sixth month (Ulūlu) of 30 days once in the 19-year cycle and a second twelfth month, Adarru, of 30 days six times in the cycle. In Mesopotamia, the beginning of the month was based on the first sighting of the new moon.

The Babylonians celebrated their New Year with a multi-day festival called the Akitu festival, a spring festival in which the king’s divine right to rule was ritually reaffirmed. The Autumn New Year is understood to be a later development in the Persian Empire in the 6th century BCE and in the Macedonian and Seleucid empires (late 4th century to early 2nd centuries BCE).

The Jewish calendar

The different features in the later Jewish calendar include alternating 29 and 30 day months, rather than the beginning of months based on observation of the moon. The luni-solar month names in the late Hebrew and Jewish calendar are the same as those in the Babylonian calendar in Aramaic forms.

The Jewish calendar adopted not only the month-names but also the structure of the year from the Seleucids. The Jewish civil year begins on Nisan 1, the first lunar month. It is not a biblical festival or marked in any ritualistic way. In Exod 12:2 God tells Moses to count the first month of “Abib” (the spring) as the first month of the year. In later and contemporary Judaism, the liturgical Jewish New Year is by custom celebrated on Tishri 1, the seventh lunar month. The Bible does not specify that this date should be New Year. There is a medieval evidence that there was a Palestinian Jewish liturgical cycle that commenced in Nisan.

Calendrical variations in the Hebrew Bible

The various solar and luni-solar calendars that appear in the Hebrew Bible with dates or as calendrical references in different forms may be ordered as follows:

1. (a) Numbered months and years according to an event or the regnal year of a king: with the day of the month, for example, Num 1:1, and 2 Kgs 25:1, and (b) without a day of the month, for instance, Ezra 3:8. Edwin R. Thiele showed that there were different methods for calculating the length of the reigns of the Israelite kings, and that the biblical computation for the beginning of the accession year was different for kings in the southern kingdom (Judah) and those in the northern kingdom (Israel).

2. The day of the month with the month given by number rather than name, showing when festivals should be observed, such as Lev 23:5. In the case of the festival of Weeks, Lev 23:15-16, the period of 50 days from the “day after the Sabbath” differed between Jewish groups, based on differing interpretations of the “Sabbath.” Here, the festival calendars varied but the calendar remained the same. The year in these cases always begins in the spring with the month of Nisan when Passover is celebrated.

3. In the later biblical books written in the Persian period (between 538 BCE and 333 BCE) the Aramaic translation of Babylonian month names are used mostly in conjunction with the numbered months, denoting the Mesopotamian calendar. These are Nisan, Sivan, Elul, Kislev, Tebeth, Shebat, Adar; they are found in Ezra 6:15; Neh 1:1; 2:1; Zech 1:7; 7:1; and several times in Esther, in chapters 2, 3, 8, 9. There is a spring new year also in these cases.

4. Non-Babylonian month names, sometimes without a number, are used in connection with the date of Passover: Abib, the first month, at Exod 13:4; 23:15; or in relation to the construction of Solomon’s Temple. The month names are here clarified by the ordinal month number: Ziv, the second month, at 1 Kgs 6:1, 37; Ethanim, the seventh month, at 1 Kgs 8:2, and Bul, the eighth, at 1 Kgs 6:38. These names are probably those used in Judah before the exile. None of these dates include the day of the month.

5. A calendar with a time scale is being followed without any ordinal or named reference to calendrical units at all, for example: Gen 29:14b-30. This concentration of time data contains the only reference in the Hebrew Bible to a week (Gen 29:27-28).

Post-biblical evidence

In post-biblical literature, such as the Book of Jubilees and the Hebrew calendars in the Dead Sea Scrolls, the ordinal month names appear in a fixed 364-day year solar calendar. It is not astronomically solar and would require intercalations so that the festivals aligned with the seasons. This calendar is exactly 52 weeks, composed of four seasons of 91 days each. As in a timetable all the festivals fell on the same day of the week every year. It began its year on the fourth day of the week, Wednesday, the day of the creation of the heavenly luminaries (Gen 1:14-19).

In the fragmentary luni-solar Aramaic calendars at Qumran only the ordinal numbers of the days of the lunar month have survived. This calendar is different to the Hebrew calendars in the Dead Sea Scrolls. It does not have days of the week and it has Mesopotamian-Hellenistic calendrical features. The calendars include the long Aramaic version of the calendrical section in the astronomical chapters of 1 Enoch, which is corrupted in the Ethiopic text.

The calendar of the Flood story

In her book, The Date of the Last Supper (La date de la cène, 1957; English translation, 1965), Annie Jaubert showed that the calendar of Noah’s flood (Gen 7:4-8:18) followed the fixed 364-day solar calendar. This was confirmed independently in the Hebrew Qumran scroll, 4Q252, published after her death, in which the biblical Flood narrative is aligned with the fixed 364-day year calendar. Jaubert also established that the biblical authors did not allow the patriarchs to travel on the Sabbath.

Further reading

Jaubert, Annie. The Date of the Last Supper. Translated by Isaac Rafferty. Staten Island, NY: Alba House, 1965.
Jacobus, Helen R. “Flood Calendars and Birds of the Ark in the Dead Sea Scrolls (4Q252 and 4Q254a), Septuagint and Ancient Near East Texts.” Pages 85-112 in Opening Heaven’s Floodgates: The Genesis Flood Narrative, Its Context, and Reception. Edited by Jason M. Silverman. Piscataway, NJ: Gorgias Press, 2013.
Stern, Sacha. “Biblical Calendars.” Pages 2014-24 in The Jewish Study Bible. Second edition. Edited by Adele Berlin and Marc Zvi Brettler. New York: Oxford University Press, 2014.
Thiele, Edwin R. The Mysterious Numbers of the Hebrew Kings. New Revised Edition. Grand Rapids, MI: Kregel Publications, 1983.