Or HaChaim on Exodus 9:5/אור החיים על שמות ט׳:ה׳

1 וישם ה׳ מועד לאמור, G'd appointed a set time, etc. He informed the Egyptians of when the plague would strike to enable them to bring their livestock indoors as we also find when the plague of hail struck (9,19). In that instance G'd told the Egyptians outright to bring their animals indoors to protect them, whereas here He only hinted at this. The hint is expressed in the Torah by the word לאמור. This meant that Moses and Aaron were to inform both Pharaoh and the Egyptians of the timing of that plague.