In the year 921/2, the Jewish leaders of Palestine and Babylonia disagreed on how to calculate the calendar. This led the Jews of the entire Near East to celebrate Passover and the other festivals, through two years, on different dates. The controversy was major, but it became forgotten until its late 19th-century rediscovery in the Cairo Genizah. Faulty editions of the texts, in the following decades, led to much misunderstanding about the nature, leadership, and aftermath of the controversy. In this book, Sacha Stern re-edits the texts completely, discovers many new Genizah sources, and challenges the historical consensus. This book sheds light on early medieval Rabbanite leadership and controversies, and on the processes that eventually led to the standardization of the medieval Jewish calendar.