The earliest known Maḥzor Vitry manuscript, MS ex-Sassoon 535, includes a section with astrological and calendar texts that are the oldest found in any extant manuscript from Franco-Germany. An edition of the text of this section, whose main part constitutes an almost complete treatise on the Jewish calendar, is offered, along with an annotated translation. The introduction includes a description of the manuscript, a summary of the contents, and some general comments on the arithmetic content of the calendar treatise and on its talmudic and other rabbinic citations. The text of the treatise is full of errors, some of which indicate that it was a copy of a copy of the original. On the basis of the calendrical data and palaeographical and codicological criteria, the earliest date for the composition of the calendar treatise can be established as 1123/4, and the latest date for the present manuscript as 1154/5. This shows that the treatise was copied repeatedly in a relatively short period. Clearly distinct from the contemporary calendar treatises by Jacob b. Samson (1123/4) and Samuel b. Meir (Rashbam, 1129/30), it provides further evidence of the developing interest in Jewish calendar computation in northern France in the early twelfth century, especially within the school of Rashi.