It was the tenth day of Nissan—40 years to the day since the Passover lambs had been killed in Egypt. Joshua and the leaders of Israel were calling the people to prepare to cross over the Jordan River, which was overflowing its banks. The people were to watch for the priests to carry the ark of the covenant before them, then they were to follow 1,000 yards behind—far enough back where the people at the front wouldn’t obstruct the view of the people in the back. 40 years after God freed you from Egypt, He was about to do something spectacular to take you into the promised land.
That’s the framework of this story. It takes up a large portion of Joshua even though all that happens is crossing a river and piling 12 stones on top of each other. But as is the norm, the author constructs in a very particular way to lead us to some theological conclusions. He even pauses the climatic part of the story, the people crossing the river, to tell us about 12 stones heaped into a mound before he backups up the story and finishes having the people cross the Jordan.