The months were reckoned by the Jews from the moon. From new moon to new moon is about 29½ 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 11¼ 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. Exodus 12:2; Exodus 13:4. This agrees with the sacred order, and all through the O.T. 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, Esther 1:3; Esther 8:12; Esther 9:1, Zechariah 1:7; Zechariah 1:7. 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.
Table of Jewish Months
Civil
Year
Sacred
Year
Name
7
1
Abib or Nisan 30 days
8
2
Zif or Iyar 29 days
9
3
Sivan 30 days
10
4
Tamus 29 days
11
5
Ab 30 days
12
6
Elul 29 days
1
7
Ethanim or Tisri 30 days
2
8
Bul or Marchesvan 29 days
3
9
Chisleu 30 days
4
10
Tebeth 29 days
5
11
Sebat 30 days
6
12
Adar 29 days