9:5 set a time. The plague has not come by coincidence. The biblical account gives no room for naturalistic explanation (such as an epidemic of anthrax stemming from dead frogs).
Tomorrow. The day’s notice suggests that God provided time for God-fearing Egyptians to put their livestock in a place of shelter (cf. vv. 18–19). Not all the Egyptian livestock succumbed to this plague (vv. 9, 19).