Albert Barnes Commentary Numbers 9:12

"they shall leave none of it unto the morning, nor break a bone thereof: according to all the statute of the passover they shall keep it." — Numbers 9:12 (ASV)

According to all the ordinances – that is, those relating to the Passover lamb, not those concerning the feast. For the Little Passover lasted, according to the Jews, only one day; nor was it considered necessary that leaven should be removed from the houses during it.