1)
Why did Hashem set a date for the plague's arrival?
1.
Maharal: Refer to 9:18:1.01:2 (regarding Makas Barad; which he contrasts with Arov, Dever and Arbeh).
2)
Why did Hashem fix the time of the plague for 'tomorrow,' and not for today?
1.
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
2.
Oznayim la'Torah: To give Pharaoh a chance to do Teshuvah.
1
Oznayim la'Torah. In fact, a Mofes with a time-tag serves also as an Os which strengthens the Mofes. See Oznayim la'Torah.
