Why did Moshe say "tomorrow" and not 'today?
Ramban: He took his cue from Par'oh, who said "tomorrow" by the plague of frogs.
Rosh (on Pasuk14): Some of the beasts go slowly, so it required a day to leave Mitzrayim.
Refer also to 9:5:1:1 & 2.
Refer to 8:6:1