Exodus 9:5 - Joseph Benson’s Commentary on the Old and New Testaments

The Lord appointed a set time This appointing of a set or particular time, both for bringing on the plagues and removing them, and that at as short a distance as the nature of the appointment would admit, and the leaving it once, at least, to Pharaoh himself to fix it, seems to have been intended to prevent the Egyptians, who were possessed with highly superstitious notions of the influence of the heavenly bodies at particular times, from thinking that Moses took advantage of his knowledge of those times to work his miracles.