John Calvin on Deuteronomy 4:10

10. The day “In the day,” etc. — lat. that thou stoodest. The word day might be taken in the accusative, as if in apposition. It is, at any rate, clear that he explains more fully what he had briefly alluded to before, for he summons the people as eye-witnesses, lest, perchance, they should object that they were not sure from whence Moses had derived what he professes to be enjoined him by God. For they were all well aware that he had undertaken nothing without the express command of God. Finally, he proves, from the end and object itself of the doctrine, that God was its author, since it tended to nothing else but that God should be purely served, and that His people might be obedient, than which nothing can be imagined more just and right.