Albert Barnes Commentary Numbers 9:6

"And there were certain men, who were unclean by reason of the dead body of a man, so that they could not keep the passover on that day: and they came before Moses and before Aaron on that day:" — Numbers 9:6 (ASV)

Certain men - Probably Mishael and Elizaphan, who buried their cousins, Nadab and Abihu, within a week of this Passover (Leviticus 10:4–5).