The day after the Passover, on that very day, they ate unleavened bread and roasted grain from the produce of the land.
11. And they did eat of the old corn, etc Whether they then began first to eat wheaten bread is not very clear. For they had dwelt in a country that was not uncultivated, and was tolerably fertile. At least in the territories of the two kings there was enough of corn to supply the inhabitants.
We may well imagine that the people of Canaan were astonished, and that when they observed the motions of the enemy they could not but think them very strange. When soldiers take the field they are apt to think themselves excused from religious exercises (they have not time nor thought to attend to them), yet Joshua opens the campaign with one act of devotion after another.
Commenting on Joshua 5:10-12
And they did eat the old corn of the land,.... That of the last year, as some versions (g), which agree with ours; in which they seem to follow the Jewish writers, who, as particularly Kimchi, Gersom, and Ben Melech, interpret it of the old corn, for this reason, because they might not eat of the new until the wave sheaf was offered up, Lev...