The names that we use nowadays come from Bablyonia (Talmud Yerushalmi Rosh Ha-Shanah 1:2); in Biblical times the months had other names.
Only four of the original Hebrew names of the months are known: Aviv
(Ex.13:4,23:15,34:18 and Deut.16:1), Eisanim (1 Kings 8:2), Bul and Ziv (1 Kings 6:1,37-38). The months are usually referred to by number rather than by name, probably because many of their names had pagan significance.
The Palestinian Talmud (Rosh ha-Shanah 1:2) says that the names of the
months that are now used were adopted by the Jews during the Babylonian Exile. Seven of these names are found in the post-Exilic books of the Bible: Nisan (Nehemiah 2:1, Esther 3:7), Sivan (Esther 8:9), Elul (Nehemiah 6:15), Kislev (Zechariah 7:1, Nehemiah 1:1), Teves (Esther 2:16), Shevat (Zechariah 1:7), and Adar (Esther, in many places). Perhaps, at the time these names were adopted, their pagan origins were no longer obvious.
The original names of the months are briefly mentioned in the articles
CALENDAR and YEAR in the Encyclopedia Judaica. More detail can be found in the article CHODESH in the (Hebrew) Biblical Encyclopedia.
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