“And it came to pass at midnight.” Here the record changes from instruction to historical event. Exodus 12:1-28 had occurred before the tenth of Nisan (the first month) and were instructions of what Israel was to do: select a lamb or goat on the tenth (Exod. 12:3, 5), kill it on the fourteenth and put the blood on the doorposts (Exod. 12:6-7), and Yahweh would pass through on the fifteenth (Exod. 12:12; the fifteenth started at sunset). Exodus 12:29 switches to the fifteenth of Nisan and Yahweh moving powerfully against Egypt.
“midnight.” The Passover Lamb was killed on Nisan 14, and at sunset the fourteenth changed to the fifteenth, (the Jewish day started at sunset, in most of the Western World the day starts at midnight) so the Israelites left Egypt on Nisan 15. The fifteenth of the lunar month was always a full moon, so the Israelites had plenty of light to travel by when they left that night.