Then Joshua told the people, “Consecrate yourselves, for tomorrow the LORD will do wonders among you.”
5. And Joshua said, etc Some unwonted manifestation of divine power in bringing assistance behooved to be held forth, lest the backwardness arising from hesitancy might produce delay; and yet, in order that the Israelites might depend on the mere counsel of God, Joshua does not yet plainly point out the special nature of the miracle, unless, indeed, we choose to read what follows shortly...
Rahab, in mentioning to the spies the drying up of the Red Sea (Jos 2:10), the report of which terrified the Canaanites more than anything else, intimates that those on that side the water expected that Jordan, that great defence of their country, would in like manner give way to them. Whether the Israelites had any expectation of it does not appear.
Commenting on Joshua 3:1-6
And Joshua said unto the people,.... On the third day; and the thirtieth day of the mourning for Moses, Jarchi says, was the first of the three days, and that being the seventh day of the month, this must be the ninth, as it is most clear the morrow was the tenth: sanctify yourselves; in a ceremonial sense, by washing their bodies and their clothes...