John Gill Commentary Exodus 9:5

"And Jehovah appointed a set time, saying, To-morrow Jehovah shall do this thing in the land." — Exodus 9:5 (ASV)

And the Lord appointed a set time
For the coming of this plague, that it might plainly appear it came from him, and was not owing to any natural cause: saying, tomorrow the Lord shall do this thing in the land ;
thus giving him time and space, as he had often done before, to consider the matter well, repent of his obstinacy, and dismiss the people of Israel, and so prevent the plague coming upon the cattle, as threatened.