Ibbronot are treatises for calculating the Hebrew calendar and Jewish holidays. In antiquity the Jewish calendar was determined by direct observation of the moon, and the calculations a closely guarded secret by the Sanhedrin, the Jewish governing body. But by the sixth century C.E., with Roman rule threatening Jewish institutions in the Land of Israel, the patriarch Hillel II released the information so that Jews the world over could celebrate the holidays at the same time. This example of ibbronot was probably written in Eastern Europe in the seventeenth century. The dense calculations common to the genre are augmented here by the use of collage-like tables or volvelles made of circles revolving around an axis attached to both sides of the page.