The generally accepted story of Rosh Hosanah is that it means “head of the year” and it falls in the 7 th month of the Hebrew Calendar. The reason given is that the seventh month (Tishrei) is, according to Jewish belief, when God began Creation. The first month, Nissan, was when the Jews were liberated from Egypt.
However, initially for the first 1,500 years of their existence, the Hebrews have always belived that Creation began at the Vernal Equinox. And the beginning of Spring, each year, celebrates that act of the Creator. The idea that Creation began in the Fall, therefore, is a mistake caused by an attempt to harmonize the concept of the first month beginning with the new moon after the Vernal Equinox, with another Calendar whose first month began in the Fall.
The people we call “Jews”, today, are those of the Diaspora. The break in the History of the Hebrews began during the exile of the leaders of the Jews to Babylon. This took place in 598 B.C. The displacement and scattering of the Jews around the Near East and the Mediterranean was a major force in shaping Judaism as we know it. The Rabbis who studied and transmitted their history strove to keep the dispersed people separate from the Gentiles: hence these are the “separated ones” or “Pharisees”.
The Hebrew name for the first month of the year ( that is, the first New Moon after the Vernal Equinox ) is “Abib”, which means (month of the Barley Harvest).
According to Nahum Sarna (Commentator on the Book of Exodus for the Jewish Publication Society in 1991), most of the Hebrew months were simply designated by numerals. We do know the names of several other months of the Hebrew Calendar (Ziv, Ethnim, Bul) but the names used by the Jews of today are really Babylonian, adopted by the Rabbis who kept the Hebrew History alive for the Jews of the Diaspora.
Since, the second Temple was destroyed by the Romans in 70 AD and the Jewish people exiled for the second time, the Jewish traditions are those of the Diaspora which was really transmitted by the Rabbis. Thus the idea that Rosh Hoshana is in the Babylonian month of Tisherei instead of the first month of the Hebrew year, is an erroneous accomodation to the Babylonian calender of the culture in which they lived.