John Calvin Commentary Numbers 9:1

"And Jehovah spake unto Moses in the wilderness of Sinai, in the first month of the second year after they were come out of the land of Egypt, saying," — Numbers 9:1 (ASV)

And the Lord spake unto Moses. We may infer how great the carelessness, indeed, even the ingratitude of the people was, from the fact that God reminded them of the celebration of the Passover before a year had passed. For what would they do fifty years from now, if there was any danger of them forgetting it in so short a time? If they had been voluntarily diligent in their duty, it would have been unnecessary to repeat what had been so severely commanded, even with threats.

But now God, as the year came to a close, reminds them that the day He had appointed for the Passover to be held was approaching. This was so that the Israelites might learn more certainly that this solemn sacrifice was an annual recurrence, and thus that omitting it was sinful.

He then commands that all the ceremonies should be diligently observed, and that they should not corrupt the pure institution with any foreign leaven. Finally, their obedience is praised, because they had neither added anything to, nor taken anything away from, God’s command.