Albert Barnes Commentary Leviticus 16:1

"And Jehovah spake unto Moses, after the death of the two sons of Aaron, when they drew near before Jehovah, and died;" — Leviticus 16:1 (ASV)

The reference to the death of Nadab and Abihu is a notice of the occasion on which the instructions were given, well calculated to add point and emphasis to the solemn admonition to the high priest in the second verse. The death of his sons (Leviticus 10:2), for drawing near to Yahweh in an unauthorized manner, was to serve as a warning to Aaron himself never to transgress in this respect.