Why did Hashem set a date for the plague's arrival?
Maharal: Refer to 9:18:1.01:2 (regarding Makas Barad; which he contrasts with Arov, Dever and Arbeh).
Seeing as Hashem is speaking, why does the Torah write "Machar Ya'aseh Hashem" in the third person?
R. Bachye #1: 'Hashem instructed Moshe Moshe to inform Yisrael that Makas Dever would take place on the following day'.
R. Bachye #2 (according to Kabalah): It means that Hashem together with his Beis-Din would bring Makas Dever. 1
See R. Bachye.
Why did Hashem fix the time of the plague for 'tomorrow,' and not for today?
Rashbam: To impress upon Pharaoh that the plague was the act of Hashem, and not a natural phenomenon that happened to have taken place just then. 1
Oznayim la'Torah: To give Pharaoh a chance to do Teshuvah.
Oznayim la'Torah. In fact, a Mofes with a time-tag serves also as an Os which strengthens the Mofes. See Oznayim la'Torah.
