Q. Why do we today use different names for the Hebrew months than those in the Bible?
A. Originally Nisan was Aviv, the month of spring. Other biblical names include Ziv, the month of flowers (I Kings 6:1), Etanim, the month of perennial streams – literally, the month of mighty ones (I Kings 8:2) and Bul, the rain month (I Kings 8:38).
These names appear to have been displaced during the Babylonian period, but we cannot be sure.
Long after the references cited above, the post-exilic prophet Zechariah (8:19) simply refers to the fasts of the 4th month (Tammuz 17), the 5th (Av 9), the 7th (Tzom Gedaliah in Tishrei) and the 10th month (Tevet 10).
Post-Babylonian era Megillat Esther already uses the names with which we are more familiar, such as Sivan and Adar, which we are told were brought up from the Babylonian exile.