I have a crazy question for all of you. My book about the Oracle of Delphi is about to go to press and I think I may have a major error in it. Most accounts of the battle at Thermopylae say the battle started August 8th or September 8th. I think I may have gotten confused by the fact that the different city-states use different calendars. If I understood Robert Hannah in his book Greek and Roman Calendars, then their calendar starts at the summer equinox and therefore August would be Boathoos...but now I'm not so sure. Any thoughts anyone??? I could really use your help.
This is actually in the FAQ! The last link is the one you want.
Briefly, the answer is that we don't know. Herodotos is our only testimony on the subject, and he's vague and simply puts it a bit after midsummer. Most modern historians tend to shift it rather later; I've found out since I wrote the post in that FAQ thread that this is because ancient Greek seasons were named for the point when the season began, so "midsummer" could mean anywhere up to a couple of months after the actual summer solstice.
I don't pretend to be an expert on this specific topic, mind -- I just know where to go to find articles on the subject -- so there's certainly room for more input here.