Komentaryo sa Exodo 9:5: Rashbam, Or HaChaim, Rabbeinu Bahya & Malbim

מועד לאמר, Moses announced the timing of the plague so that the Egyptians could not claim that the plague was a natural disaster.

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.

וישם ה' מועד לאמור מחר יעשה ה' הדבר הזה בארץ, “G’d appointed a set time, saying: “G’d will carry out this matter in the land tomorrow.” Why this cumbersome language? All the Torah had to write was: “Tomorrow I will do it.” The plain meaning of this verse is that G’d gave Moses the date on which this plague would occur and he was to tell the Israelites the date, i.e. on the following day.