Joshua 10:9 mentions that he came upon them suddenly after a night march. It would have taken the army of Israel around eight or ten hours to travel to Gibeon from Gilgal.1 The troops would have most certainly been tired by the time they reached the city. With no time to prepare, they suddenly came upon the armies of the Amorite kings. Most likely this would have been at dawn, just before sunrise.
9 So Joshua came upon them suddenly, having marched up all night from Gilgal.