In today's review of Ephilates, we talked about how he betrayed the Spartans. But when did he actually do it?
The traditional date for the Battle of Thermopylae is August 11, but that's almost certainly incorrect. The Immortals traveled the mountain path by night, surprising the Phocian guards stationed there at dawn on the third morning. Since they were unfamiliar with the local terrain, the Persians would have needed as much light as possible, but torches would have given them away: so they had to be relying on the light of the moon. Take a look at a lunar calendar for August, 480 BC, and you'll see that the first quarter was on the 12th (a Sunday), which means the night would still have been quite dark. Judging solely by the moon, it seems likely the battle actually look place later in the week.
Some amateur historians try to place the battle in September. The reason the full Spartan army couldn't go to Thermopylae was that the city was in the midst of the Carneia festival - the same reason they weren't at the battle of Marathon a decade earlier. So folks reason "Marathon was in September, the festival must be in September." Nope.
Unsurprisingly, ancient Greeks used a different calendar than we do - the Hellenic festival calendar was lunar, starting on the first new moon after the summer solstice each year. The Carneia began on the seventh day of the second month, and ran for nine days after that - putting its end right near the full moon. In 480 BC, the month would have begun on August 5, so the festival wouldn't even be going by the 11th. Taking into account all the evidence, it's more likely that the Battle of Thermopylae took place around August 16th, as we reckon things.