Mishneh torah: 6v-7r
Moses Maimonides' Code of Jewish Law was the earliest universally accepted Jewish legal code. Prior to his work, legal rulings could only be inferred through intensive study of the Talmud and its surrounding texts. The third volume of his Code deals with time, including the Jewish festivals and the setting of months. The astronomical details discussed in Maimonides' code were accepted universally, and remain the basis for the modern Jewish calendar.
The opening shown here is the beginning of the section that discusses the laws for various Jewish holidays.