Chodesh from chadash, "new," namely, new moon; chodesh yamin "a month of days" (Genesis 29:14); also the poetical yerach from yareach "the moon," so month is connected with moon in European languages; German mond and monat; Greek meen, mene; Latin, mensis; Sanskrit, masa, both "moon" and "month". The interval between the 17th day of the second month (Genesis 7:11) and the 17th day of the seventh month is said to be 150 days (Genesis 8:3-4), i.e. five months of 30 days each; thus the year would be 360 days, corresponding to the old Egyptian year, possibly too five days were intercalated to complete the 365 of the solar year; at all events there is an approximation to the solar year. The total duration of the flood was eleven days above a year (Genesis 7:11; Genesis 8:14), the exact excess of the solar year above the lunar of 354 days. Genesis 1:14; Genesis 1:16 harmonizes with the theory of a double year, solar and lunar.
The Passover depended on the moon, the 14th of Abib coinciding with full moon. The new moon was a regular feast day (Numbers 10:10; Numbers 28:11-14). Latterly, its appearance (which may be seen 40 hours after the moon's conjunction with the sun) was reported by proper witnesses to the authorities, who announced the month's commencement by twice repeating "mequdash," consecrated. Modern Jews observe the lunar month. Its length would be alternately 29 (a "deficient month," chasar in the Talmud) and 30 days ("full month," malee'). The seasons regulated the months, e.g. Abib the first month of the year was that of "ears of grain"; in the Passover in it, on the second day, the sheaf of harvest firstfruits was waved to the Lord (Leviticus 23:10-12-34-39; Joel 2:28). So the feast of tabernacles in the seventh month celebrated the ingathering of the autumnal fruits; so that a solar year must have regulated the months.
The months were 12 (1 Kings 4:7), with an intercalary month every third year, not noticed in the Bible. The modern Jews have seven intercalary months in every 19 years, according to the metonic cycle adopted A.D. 360. Four names of months are mentioned before the Babylonish captivity: Abib ("the month of ears of grain") made the first month in memory of the Exodus (Exodus 9:81; Exodus 12:2; Exodus 13:4); Zif ("the bloom of flowers", or the Assyrian gay, "bull," the zodiacal Taurus), the second month (1 Kings 6:1; 1 Kings 6:37); Bul ("the month of rain"), the eighth month (1 Kings 6:38); Ethanim ("the month of gifts", namely, fruits), the seventh (1 Kings 8:2). The three latter names are found only in Solomon's reign, when there was much intercourse with Phoenicia; they are probably Phoenician in origin. "Bul" is mentioned on a sarcophagus found near Sidon in 1855. They are explained by the addition "which is the" second, the eighth, the seventh month.
After the captivity the first month (that of the Passover) was called Nisan (Nehemiah 2:1); Sivan the third (from the Assyrian siv "the moon", to whom the Assyrians consecrated it): Esther 8:9. Elul the sixth (Nehemiah 6:15); Chisleu the ninth (Nehemiah 1:1); Tebeth (from the Egyptian tobi) the tenth (Esther 2:16); Sebat the eleventh (Zechariah 1:7); Adar the twelfth (Esther 3:7). The Talmud gives the remaining five: Iyar the second, Tammuz the fourth (sacred to that idol), Ab the fifth, Tisri the seventh, Marchesvan (from mar "to drop") the eighth; mainly named from the Syrian calendar. The intercalary month was Veadar, i.e. the additional Adar. The variations between the lunar and the solar month, each of the lunar ranging over two solar months, prevent exact coincidence with our months. The barley harvest is not until the middle of April, so that Abib or Nisan, in which the Passover first sheaf was offered on the 15th day, coincides with April. Josephus (Ant. 3:10, section 5) says the Passover was while the sun is in Aries, which it does not enter until the end of March. Zif or Iyar is May, Sivan is June, Tammuz is July, Ab is August, Elul is September, Ethanim or Tisri is October, Bul or Marchesvan is November, Chisleu is December, Tebeth is January, Sebat is February, Adar is March.
Fausset, Andrew Robert M.A., D.D., "Definition for 'month' Fausset's Bible Dictionary".