Albert Barnes Commentary Exodus 11:4

"And Moses said, Thus saith Jehovah, About midnight will I go out into the midst of Egypt:" — Exodus 11:4 (ASV)

And Moses said - The following words must be read in immediate connection with the last verse of the preceding chapter.

About midnight - This marks the hour, but not the day, on which the visitation would take place. There may have been, and probably was, an interval of some days, during which preparations might be made both for the celebration of the Passover, and the departure of the Israelites.