We don't know the actual date of the Battle of Marathon, but September 12 was suggested by one historian in 1855, and everyone seems to have just rolled with it.
On a day in 490 BC which might have been this one, some guys won a very historically-important battle by running at top speed in full armor a mile and a half downhill[disputed]. While this is bad-ass, the thing that people remember most is the one guy who ran another 26 miles to deliver the news to the city of Athens. What's even worse, most people forget that guy had just finished running 150 miles to Sparta to ask for reinforcements, which were not provided. Which is why he immediately died of exhaustion.
Join the celebrations by playing one of the Marathon games. While the Halo franchise arguably lives in the same universe, the Battle of Marathon was fought without the help of any Spartans, so you should restrict yourself to ODST.