At what time did the LORD strike down the firstborn of Egypt?

Answer: At midnight

Moses warned, "About midnight I will go throughout Egypt," and at midnight the LORD struck down every firstborn.

“So Moses declared, “This is what the LORD says: ‘About midnight I will go throughout Egypt, Now at midnight the LORD struck down every firstborn male in the land of Egypt, from the firstborn of Pharaoh, who sat on his throne, to the firstborn of the prisoner in the dungeon, as well as all the firstborn among the livestock.”— Exodus 11:4, 12:29 (BSB)