Cairo Genizah fragment, from a late eleventh-century book containing copies of letters written by Rav Saadya Gaon in 921-923, in Baghdad, to his former disciples in Fustat (Cairo). The letters are in Hebrew and Judeo-Arabic; but at the end of this letter (on the left), the names of the addressees are written in Arabic.

In these letters, Saadya reprimands his disciples for observing the Jewish festivals on the wrong dates. In the years when the letters were written, the Rabbanite Jews of Palestine and Iraq disagreed about the dates of the new moons and festivals. Saadya's disciples, in Egypt, were misled by the early appearance of the new moon crescent, and as the result, they sided with the Jews of Palestine. This episode illustrates the complex relationship between the religious calendar and the rhythms of nature.

Other folios of this manuscript are currently held in the Bodleian Library, Oxford, and in the Cambridge University Library.