Summary This paper is concerned with a mechanical calendar described by the great scientist al-B?r?n?, who died in 440/1048. The description occurs in a book devoted to the construction of various types of astrolabe and related instruments. The Arabic text presented in this paper was prepared from three manuscripts. This is preceded by a brief introduction which gives a sketch of the life and works of al-B?r?n? together with information about the provenance and contents of the three manuscripts. The text (...) is followed by an English translation and the paper concludes with a technical commentary which discusses the construction and operation of the instrument. This commentary is accompanied by explanatory drawings reconstructed from al-B?r?n?'s descriptions. Photographs from the illustrations in the manuscripts are also provided. (shrink)