“Say to the people: Make yourselves holy for tomorrow, and you will eat flesh; for you have wept in the ears of Yahweh, saying, ‘Who will give us flesh to eat? For it was well with us in Egypt.’ Therefore Yahweh will give you flesh and you will eat.

“make yourselves holy.” The people were to do what it took to make themselves holy in the sight of God (cp. Lev. 11:44).
[For more on “make yourselves holy,” see commentary on Joshua 3:5.]