Carneia

The festival of Apollo Carneus, and the reason Sparta sent three hundred men to Thermopylae instead of an army. Herodotus states it directly: the Spartans meant to march in force, but the Carneia was in the way, and they sent an advance guard intending to follow once the festival ended.

The same thing had happened a decade earlier. Herodotus reports that before Marathon the Spartans would not march until the moon was full, and arrived after the fighting was over.

This is the fact that most often gets removed from retellings, because it complicates the picture. Thermopylae was not a suicide mission undertaken by choice. It was a holding action by a covering force whose relief was waiting on a religious calendar.