Battle of thermopylae Question.?

Persia sent messengers to all of the small countries with only one city (also known as a city-state) stating that if they did not join the Persian army they would be destroyed.The Spartan king Leonidas tried to train his army up knowing that since they had denied this the Persians would be after them. The Spartan state council stopped Leonidas doing so, and stated that no one shall declare war during a certain holiday (Which holiday is this ??????!!!!!)