Numbers 8:17-18

8:17 For all the firstborn males among the Israelites are mine, both humans and animals; when I destroyed all the firstborn in the land of Egypt I set them apart for myself. 8:18 So I have taken the Levites instead of all the firstborn sons among the Israelites.

tn The idiomatic “on the day of” precedes the infinitive construct of נָכָה (nakhah) to form the temporal clause: “in the day of my striking…” becomes “when I struck.”