Aviv 1 - March 23
The Month - HaChodesh - is reckoned by the moon. The psalmist tells us: “Blow the shofar at the new moon, at the hiddden moon, on our feast day.” (Psalm 81:3)
A month varies between 29 and 30 days because the moon circles the earth once in approximately 29 and ½ days. Since this is an approximation, two months have to be adjusted year after year, Bul and Kislev, to achieve an accurate Molad (lunation) in one year.
“This month shall be to you the beginning of months; it shall be the first month of the year to you.” (Exodus 12:2). This passage refers to the spring month of Aviv when Passover is to occur, thus, the biblical calendar begins with Aviv.
The post biblical calendar that Hillel II produced do not follow this biblical counting of the months, but the Talmudic interpretation that the earth was created in the seventh month, the month of Tishrei in the fall, and thus the month of Tishrei is instead the beginning month of the Jewish calendar.
The Bible mention four months before the Babylonian exile: Aviv, Ziv, Etanim, and Bul, and six after the exile: Sivan, Elul, Kislev, Tevet, Shevat, and Adar. After the Babylonian exile, in the Biblical writings, Aviv was changed to Nisan, and in the Talmudic writings Ziv to Iyar, Etanim to Tishrei, and Bul to Cheshvan. There are two months that do not have names in the Bible, the 4th and the 5th. In the Talmudic writings they correspond to Tammuz and Av.
Happy New Biblical Year!