The rabbinic calendar is by far the best known of all ancient Jewish calendars. It is described at length in Mishnaic and Talmudic literature, mainly in the tractate Rosh ha-Shanah. It is also the main Jewish calendar to have survived till the present day. Yet in spite of this, the origins of the present-day rabbinic calendar are extremely unclear. After more than a century of scholarly research, greatly enhanced by discoveries in the Cairo Geniza, the history of the rabbinic calendar in the first millennium ce, and particularly from the post-Mishnaic to the later Geonic periods, remains shrouded in mystery.
Tradition has it that in 358-9 ce the rabbinic calendar underwent a radical reform. The older calendar, described in the Mishnah and based on empirical sightings of the new moon, was replaced with the fixed, calculated calendar that is in force today. Evidence of such a reform, however, is absent in contemporary sources such as the Palestinian and Babylonian Talmud. It is only first mentioned in late tenth- or early eleventh-century sources, which may cast doubt on the reliability of this tradition.
The present-day rabbinic calendar, likewise, is only fully attested and described in early tenth-century sources, in the polemical correspondence of R. Saadya and Ben Meir. There is thus a gap of about seven centuries between the Mishnah and R. Saadya's period, during which little can be said with certainty about the rabbinic calendar.
The limited evidence that is available has been studied in detail by earlier scholars, although fresh evidence continues occasionally to emerge. Among the earlier scholars, H. Y. Bornstein is worthy of special mention. Of breathtaking scope and proficiency, he published a series of articles in the first decades of the twentieth century, largely based on then recent Geniza discoveries, that had a formative influence on the modern understanding of the rabbinic calendar and its origins. Other scholars to have made significant contributions to this field, in the later