The Greek writer Plutarch, who wrote his "Life of Alexander" around 100 A.D., is practically our only source concerning the birth of Alexander the Great and its circumstances. As this is only one piece of information, its value tends to be relative. Knowledge starts when you have the same piece of information from different, independent sources. So one has to bear in mind that whatever Plutarch says could be only legend and might not be backed up by other sources.
The British scholar Nicholas Hammond made a thorough analysis of the probable dating of Alexander's birth in his "Sources for Alexander the Great" (Cambridge, 1993). Hammond shows that Plutarch probably got his date of 6th of the Attic month Hecatombaeon 356 B.C. for Alexander's birthday from a contemporary writer, Timaeus, who was also born in 356 B.C. Plutarch says Alexander was born on the same day that the Temple of Artemis at Ephesus (near Izmir in modern Turkey) was destroyed by fire. Artemis was, amongst her other attributes, the goddess of childbirth. Legend had it that this destruction by fire of this shrine (which eventually became one of the famous Seven Wonders of the ancient world) spelled doom for the peoples of Asia. There is a possibility that the dating of Alexander's birth has been manipulated by our ancient sources to coincide with this disaster. See also: Seven Wonders.
20th of July?
So, author Robin Lane Fox cites three possibilities for Alexander's birthday: July 20th 356 B.C. or thereabouts, July the 6th 356 B.C. or October 356 B.C. Basically, Cicero gives essentially the same story about Alexander's birth and the temple of Ephesus burning down as Plutarch does, and he too attributes it to Timaeus. It follows that the date of 6th of Hecatombaeon is well-sourced. Unfortunately, it is difficult to match this date exactly with the Julian Calendar (which we tend to use for ancient events). Hammond says there is general agreement for the second half of July. So, after all, 20th of July is a good guess.
Of course, Alexander's birth became surrounded by other legends as well. Stories circulated around the Macedonian court that his mother Olympias had conceived from a snake. Olympias had startling dreams too --- about a circle of fire extending to the outer reaches of the earth. And also there is a dream of King Philip II about Olympias' womb being sealed by a lion. For some pretty Medieval folk tales about Alexander's birth, see: Legends (Birth).
This short article includes contributions from Forum members Jona Lendering, Andrew Chugg and Dean.