Aaron ben Meir

In 921–922 CE, Ben Meir ignited one of the most consequential disputes of the Geonic age — a clash over who held the authority to fix the Jewish calendar. He published a new calculation of the molad (the lunar conjunction) of Tishri, by which the festivals of 922 would fall two days earlier than the reckoning used by the Babylonian academies: that year, he ruled, Passover should begin on a Sunday rather than a Tuesday, with Rosh Hashanah shifting accordingly. Beneath the technical question lay a contest of leadership — Ben Meir insisted that the calendar must be set from the Land of Israel, the ancient seat of the Sanhedrin, and denied the Babylonian centers any authority in such matters.

The Babylonian leadership resisted fiercely. The Exilarch David ben Zakkai and the academies rejected his calculation, and a young Saadia ben Yosef — soon to become Gaon of Sura — emerged as the decisive opponent, marshaling the mathematical and halachic case against him and composing his Sefer ha-Mo'adim ('Book of the Festivals') to refute it. For two years parts of the Jewish world kept the festivals on different days. In the end Ben Meir's position failed: he was placed under a ban, circular letters were sent warning communities against his reckoning, and the Babylonian calculation prevailed. The episode is remembered as a turning point — the moment that confirmed Babylonia, not the Land of Israel, as the arbiter of Jewish practice for the centuries that followed, and that launched the public career of Saadia Gaon.