וַיְהִ֣י ׀ בַּחֲצִ֣י הַלַּ֗יְלָה וַֽיהוָה֮ הִכָּ֣ה כָל־בְּכוֹר֮ בְּאֶ֣רֶץ מִצְרַיִם֒ מִבְּכֹ֤ר פַּרְעֹה֙ הַיֹּשֵׁ֣ב עַל־כִּסְא֔וֹ עַ֚ד בְּכ֣וֹר הַשְּׁבִ֔י אֲשֶׁ֖ר בְּבֵ֣ית הַבּ֑וֹר וְכֹ֖ל בְּכ֥וֹר בְּהֵמָֽה׃
And it came to pass at midnight, that the LORD smote all the firstborn in the land of Egypt, from the first-born of Pharaoh that sat on his throne unto the first-born of the captive that was in the dungeon; and all the first-born of cattle.
Rashi on Exodus
וה׳ “AND” THE LORD — Wherever it is stated וה׳ “And the Lord” it signifies He and His celestial Court, because the prefix ו expresses an addition, just as one says, “Mr. So-and-so and Mr. So-and-so” (Genesis Rabbah 51:2; cf. Rashi on Genesis 19:24).
Sforno on Exodus
וה' הכה, while the Jews were busy with preparing and eating the Passover offering, G’d was busy with killing the firstborn of Egypt and orchestrating the redemption of His people.
Rashbam on Exodus
בחצי הלילה, while they were eating their Passover offering.