We do not have an accurate date. Herodotus just states: “not many years before the King's expedition, the Thessalians and their allies had invaded Phocis with their whole army …” This puts it sometime before Marathon in 490 BC.
Herodotus states further:
The Phocians had taken refuge on Mount Parnassus. Their diviner Tellias of Elis devised a clever stratagem for them. He had six hundred of the bravest Phocians completely cover themselves with chalk, along with their weap- ons and armour, and led them to attack the Thessalians by night, telling them to slay anyone they should see who was not similarly whitened like they were. The Thessalian sentries were the first to see these men and to flee for fear, supposing falsely that it was something supernatural, and after the camp guards
