From the time of the institution of the Mosaic law downward the religious feasts commencing with the passover depended not simply on the month, but on the moon; the 14th of Abib was coincident with the full moon; and the new moons themselves were the occasions of regular festivals. (Numbers 10:10; 28:11-14) The commencement of the month was generally decided by observation of the new moon. The usual number of months in a year was twelve, as implied in (1 Kings 4:7; 1 Chronicles 27:1-15) but since twelve lunar months would make but 354 1/2 days, the years would be short twelve days of the short twelve days of the true year, and therefore it follows as a matter of course that an additional month must have been inserted about every third year, which would bring the number up to thirteen. No notice, however, is taken of this month in the Bible. In the modern Jewish calendar the intercalary month is introduced seven times in every nineteen years. The usual method of designating the months was by their numerical order, e.g. "the second month," (Genesis 7:11) "the fourth month," (2 Kings 25:3) and this was generally retained even when the names were given, e.g. "in the month Zif, which is the second month." (1 Kings 6:1) The names of the months belong to two distinct periods. In the first place we have those peculiar to the period of Jewish independence, of which four only, even including Abib, which we hardly regard as a proper name are mentioned, viz.: Abib, in which the passover fell, (Exodus 13:4; 23:15; 34:18; 16:1) and which was established as the first month in commemoration of the exodus, (Exodus 12:2) Zif, the second month, (1 Kings 6:1,37) Bul, the eighth, (1 Kings 6:38) and Ethanim, the seventh. (1 Kings 6:38) and Ethanim, the seventh. (1 Kings 8:2) In the second place we have the names which prevailed subsequent to the Babylonish captivity; of these the following seven appear in the Bible: Nisan, the first, in which the passover was held, (Nehemiah 2:1; Esther 3:7) Sivan, the third (Esther 8:9) Bar. 1:8; Elul, the sixth, (Nehemiah 6:15)
1 Macc. 14:27; Chisleu, the ninth, (Nehemiah 1:1; Zechariah 7:1)
1 Macc. 1:54; Tebeth, the tenth, (Esther 2:16) Sebat, the eleventh, (Zechariah 1:7)
1 Macc. 16:14; and Adar, the twelfth. (Esther 3:7; 8:1)
2 Macc. 15:36. The names of the remaining five occur int he Talmud and other works; they were, Iyar, the second, Targum; (2 Chronicles 30:2) Tammuz, the fourth; Ab, the fifth; Tisri, the seventh; and Marcheshvan, the eighth. The name of the intercalary month was Ve-adar, i.e. the additional Adar. The identification of the jewish months with our own cannot be effected with precision on account of the variations that must inevitably exist between the lunar and the solar month. Nisan (or Abib) answers to March; Zif or Iyar to May; Sivan to June; Tammuz to July; Ab to August; Elul to September; Ethanim or Tisri to October; Bul or Marcheshvan to November; Chisleu to December; Tebeth to January; Sebat to February; and Adar to March.
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Smith, William, Dr. "Entry for 'Month'". "Smith's Bible Dictionary".