Boyd’s Bible Dictionary:

(moon). Hebrew month lunar, from new moon to new moon (Num. 10:10; 28:11-14). Intercalary month every three years. Months named, but usually went by number (Gen. 7:11; 2 Kings 25:3). See month names in place.

Concise Bible Dictionary:

The months were reckoned by the Jews from the moon. From new moon to new moon is about 29.5 days, and to suit this period their months consisted alternately of 29 and 30 days; thus 12 months would amount to only 354 days; being 11.25 days short of a solar year. This reckoning would soon have thrown out some of their festivals because they were connected with the first-fruits of the barley and wheat harvest. Every three years a month had to be added, and this was called Ve-adar, the “added Adar.” There were two periods for the year to commence: one called the Civil year, and the other the Sacred. The Israelites were brought out of Egypt in the month Abib, and that was to be the beginning of the year to them (Ex. 12:2; Ex. 13:4). This agrees with the sacred order, and all through the Old Testament when the name of a month is given, its position in the year agrees with this arrangement (See 1 Kings 6:1; Esther 3:7,13; Esther 8:12; Esther 9:1; Zech. 1:7; Zech. 7:1). It is remarkable that the Jews now begin their year on the first day of Tisri (in September), which stands the first month of the civil year.
The months of Tammuz and Ab are not mentioned in scripture. The names in italics are used by Josephus and others. See SEASONS and YEAR.
Civil Year Sacred Year
7 1 Abib or Nisan. 30 days.
8 2 Zif or Iyar.29 days.
9 8 Sivan. 30 days.
10 4 Tammuz. 29 days.
11 6 Ab. 30 days.
12 6 Elul. 29 days.
1 7 Ethanim or Tisri.30 days.
2 8 Bul or Marchesvas.29 days.
3 9 Chisleu. 30 days.
4 10 Tebeth. 29 days.
5 11 Sebat. 30 days.
6 12 Adar. 29 days.

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