When did Jews begin celebrating Rosh Hashanah on the first of Tishri rather than Nisan? Contrary to widespread belief that this shift occurred in post-exilic times, evidence points to a much later transition around 100 C.E. under Gamaliel II's leadership. While the Torah established Nisan as the sacred year's beginning and marked Tishri as a "Day of Memorial," biblical, talmudic, and historical sources reveal a complex evolution of Jewish calendrical practices. Through analysis of primary texts spanning from Torah through Mishnaic literature, references in Nehemiah, Maccabees, Josephus's writings, and Talmudic tractates, alongside comparative study of Ethiopian Falasha traditions, a pattern emerges. Josephus's accounts demonstrate the persistence of a dual calendar system, while rabbinic debates between Joshua and Eliezer highlight ongoing disagreement about new year timing. The formal establishment of Tishri 1 as Rosh Hashanah appears to have been a deliberate decision by the Bet Din at Jabneh, implemented to unite dispersed Jewish communities following the Temple's destruction. This finding significantly revises our understanding of Jewish calendar development and post-Temple religious consolidation.