Albert Barnes Commentary Numbers 9:1-5

"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, Moreover let the children of Israel keep the passover in its appointed season. In the fourteenth day of this month, at even, ye shall keep it in its appointed season: according to all the statutes of it, and according to all the ordinances thereof, shall ye keep it. And Moses spake unto the children of Israel, that they should keep the passover. And they kept the passover in the first [month], on the fourteenth day of the month, at even, in the wilderness of Sinai: according to all that Jehovah commanded Moses, so did the children of Israel." — Numbers 9:1-5 (ASV)

The Passover at Sinai, since it was kept in the first month, occurred before the numbering mentioned in Numbers 1:1 and following and the other events recounted in this book (Numbers 9:5). It is, however, recorded here as an introduction to the ordinance in Numbers 9:6-14 concerning the supplementary Passover. The observance of this supplementary Passover was one of the last events during the stay at Sinai.

In some details, this particular Passover differed both from the one kept at the Exodus itself and from all subsequent Passovers. For example, the instruction in Exodus 12:22 could not be carried out literally while the people were living in tents. This instruction may be regarded as superseded by Leviticus 17:3-6 (compare Deuteronomy 16:5 and following).

In other matters, such as how many lambs would be needed, or how the blood of the Passover sacrifices could be sprinkled on the altar within the specified time, etc., the administrators of the Law of Moses would, in this instance as in others, necessarily have the authority to order what was required to implement the law.