The first book of Macrobius' Saturnalia, written probably in the 430s AD, includes a historical exposition of the Roman calendar with a dramatic date some fifty years earlier, set in the mouth of the learned senator Vettius Agorius Praetextatus, followed by more technical detail at the request of an Egyptian named Horus, who as a foreigner is allowed to seek elementary information for which no one brought up in Roman culture would need to ask. This text was excerpted in early medieval Ireland, with some but by no means all its pagan matter excised, to provide an introduction for those who at best understood the rules of this recent import but not the rationale for them; it is quoted by Bede as Disputatio Chori et Praetextati, Chorus being a corrupted form of Horus. The excerpt took on a textual life of its own, which the present edition, the first devoted to the Disputatio rather than Macrobius, seeks to clarify; it examines the manuscripts and the relations between them, presents a critical edition with apparatus criticus and translation, and attaches a full-scale commentary concerned above all with the information provided in the text.