Why did Hashem fix the time of the plague for 'tomorrow', and not today?
Rashbam: To impress upon Par'oh 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: Also to give the Par'oh a chance to do Teshuvah.
Oznayim la'Torah. In fact, a Mofeis with a time-tag serves also as an Os which strengthens the Mofeis.See Oznayim la'Torah.