As far as I know, there is no single classical source which states categorically that JC was born on 13 Quintilis (July), the fifth month of the original calendar. It can be inferred from several sources that he was born in Quintilis. Heraklia's beloved Macrobius states, "a.d. quartum Idus Quintiles Iulius procreatus sit" (Saturnalia I, 12, 34), which adds up to 12 July, which date is followed by other sources. From Cassius Dio, 47. 18. 6, however, we learn that JC's birthday fell on the day on which the Ludi Apollinares were celebrated (i.e. 13 Quintilis/July), and that the Sibyl had forbidden there to be a festival to any other god on that day. Therefore, from 42 BC, JC's official birthday, and its attendant celebrations, were moved to the previous day, 12 July. Are you all with me? Not so much like Shakespeare as Her Majesty the Queen, who was born on 21 April but whose official birthday is in June.