Boyd’s Bible Dictionary:

(moon). Hebrew month lunar, from new moon to new moon (Num. 10:10; 28:11-1410Also in the day of your gladness, and in your solemn days, and in the beginnings of your months, ye shall blow with the trumpets over your burnt offerings, and over the sacrifices of your peace offerings; that they may be to you for a memorial before your God: I am the Lord your God. (Numbers 10:10)
11And in the beginnings of your months ye shall offer a burnt offering unto the Lord; two young bullocks, and one ram, seven lambs of the first year without spot; 12And three tenth deals of flour for a meat offering, mingled with oil, for one bullock; and two tenth deals of flour for a meat offering, mingled with oil, for one ram; 13And a several tenth deal of flour mingled with oil for a meat offering unto one lamb; for a burnt offering of a sweet savor, a sacrifice made by fire unto the Lord. 14And their drink offerings shall be half an hin of wine unto a bullock, and the third part of an hin unto a ram, and a fourth part of an hin unto a lamb: this is the burnt offering of every month throughout the months of the year. (Numbers 28:11‑14)
). Intercalary month every three years. Months named, but usually went by number (Gen. 7:1111In the six hundredth year of Noah's life, in the second month, the seventeenth day of the month, the same day were all the fountains of the great deep broken up, and the windows of heaven were opened. (Genesis 7:11); 2 Kings 25:33And on the ninth day of the fourth month the famine prevailed in the city, and there was no bread for the people of the land. (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. 12:22This month shall be unto you the beginning of months: it shall be the first month of the year to you. (Exodus 12:2); Ex. 13:44This day came ye out in the month Abib. (Exodus 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 6:11And it came to pass in the four hundred and eightieth year after the children of Israel were come out of the land of Egypt, in the fourth year of Solomon's reign over Israel, in the month Zif, which is the second month, that he began to build the house of the Lord. (1 Kings 6:1); Esther 3:7,137In the first month, that is, the month Nisan, in the twelfth year of king Ahasuerus, they cast Pur, that is, the lot, before Haman from day to day, and from month to month, to the twelfth month, that is, the month Adar. (Esther 3:7)
13And the letters were sent by posts into all the king's provinces, to destroy, to kill, and to cause to perish, all Jews, both young and old, little children and women, in one day, even upon the thirteenth day of the twelfth month, which is the month Adar, and to take the spoil of them for a prey. (Esther 3:13)
; Esther 8:1212Upon one day in all the provinces of king Ahasuerus, namely, upon the thirteenth day of the twelfth month, which is the month Adar. (Esther 8:12); Esther 9:11Now in the twelfth month, that is, the month Adar, on the thirteenth day of the same, when the king's commandment and his decree drew near to be put in execution, in the day that the enemies of the Jews hoped to have power over them, (though it was turned to the contrary, that the Jews had rule over them that hated them;) (Esther 9:1); Zech. 1:77Upon the four and twentieth day of the eleventh month, which is the month Sebat, in the second year of Darius, came the word of the Lord unto Zechariah, the son of Berechiah, the son of Iddo the prophet, saying, (Zechariah 1:7); Zech. 7:11And it came to pass in the fourth year of king Darius, that the word of the Lord came unto Zechariah in the fourth day of the ninth month, even in Chisleu; (Zechariah 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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