With the first anniversary of the Exodus approaching, the Children of Israel are commanded regarding Pesach:
And Hashem spoke to Moshe in the wilderness of Sinai, in the second year of their exodus from the land of Egypt, in the first month, saying, “And the Children of Israel will make the Pesach at its time. On the fourteenth day of this month in the afternoon shall they make it at its time, according to all its statutes and according to all its laws shall you make it.” And Moshe spoke to the Children of Israel to make the Pesach. And they made the Pesach in the first [month], on the fourteenth day of the month, in the afternoon, in the wilderness of Sinai; according to all that Hashem commanded Moshe, so did the Children of Israel do (Bamidbar 9:1-5).
What follows is the incident of those people who, due to tumah, could not offer the Pesach sacrifice. Subsequently, Moshe taught the nation about Pesach Sheni
(verses 6-14).

However, we will focus on the above admonition to offer the Pesach sacrifice in the wilderness. The Torah does not record any reminders before other festivals. Why does Hashem prompt the people only about Pesach? Moreover, according to Ramban, this command is concerned exclusively with the Pesach sacrifice, and does not at all pertain to the seven days of Pesach. (In this he disagrees with Rashi.) Why is the Pesach sacrifice singled out?

It must also be asked why the Torah emphasizes the place and precise time of this command. Note that this event antedates the opening of the book of Bamidbar. Why does the Torah choose to postpone its inclusion until nine chapters into the book?

Rashi, quoting from the Sifrei (64, 67), says that in the wilderness of Sinai, in the second year of their exodus from the land of Egypt, in the first month was the only time that the Children of Israel offered the Pesach sacrifice in their forty years in the wilderness. The event was removed from the beginning of Bamidbar so as not to call attention to this unfortunate fact.

Two approaches are found among our Sages regarding this. The Talmud (Yevamot 72a) explains that one whose children or slaves remain uncircumcised is forbidden to offer the Pesach sacrifice. In the wilderness, the people were unable to circumcise newborns or slaves, because it was dangerous: Hashem could have ordered them to travel at a moment’s notice, making recuperation uncertain; besides, there was no healing north wind throughout their stay in the wilderness. According to the Sifrei, on the other hand, the sin of the scouts (Parshat Shelach)prevented them from entering the land of Israel during the second year. Therefore, it is to their shame and discredit that they could not offer the Pesach sacrifice for 39 years.

Ramban explains why this was so. While it is true that all other aspects of the seven-day festival of Pesach apply both within the land of Israel and without, the Pesach sacrifice of the 14th day of the first month was not to go into effect until their entry into the land. As it says,
And it shall be, when you shall come to the land which Hashem will give you, as He said, that you shall keep this service (Shemot 12:25).
And it shall be, when Hashem shall bring you into the land of the Canaanite…that you shall perform this service in this month (Ibid.,13:5).
Indeed, the prophet asks rhetorically
Did you present Me with sacrifices and grain offerings for forty years in the wilderness, O house of Israel?! (Amos 5:25).

Ramban’s approach is paralleled in the Zohar (III, 151b-152a) stating that the Children of Israel thought they did not have to offer the Pesach sacrifice again, since they had already done so in Egypt. Hashem repeated the command at Sinai, the place where all the other mitzvot were given.

The Children of Israel had been commanded to offer the Pesach sacrifice on the threshold of their liberation from Egypt; this was Pesach Mitzrayim. At that time, they were also informed that they would resume this service upon their entry to the Promised Land, and continue for all generations; this was to be Pesach l’dorot. Had they never rejected the Land, then only one Pesach – “in the second year of their exodus from the land of Egypt” – would have separated Pesach Mitzrayim from Pesach ledorot. Instead, after this lone Pesach Bamidbar, there was a disgraceful 39-year hiatus.

It is clear, however, that Hashem wanted there to be a Pesach sacrifice in the wilderness. One question, therefore, remains. What is the purpose of Pesach Bamidbar?

Ramban answers:
“And now the Holy One Blessed be He desired and commanded that they make it in order that the memory of their redemption and of the miracles which were done for them and their forefathers should be transmitted from the fathers who saw them to their children, and their children to another generation (Yoel 1:3).”

In order to understand Ramban’s subtle but crucial idea, we need only put ourselves in the position of the generation of the Exodus. While making the transition from slavery to freedom, we knew we were living through a momentous, history-making event. We also knew that the time would come when we would commemorate and relive that event once our lives would be stable in our homeland. But first Hashem commanded us, “the fathers who saw,” to construct the bridge to the future with Pesach Bamidbar.

The same is true of any generation that has witnessed history being made. First comes the dizzying intensity of the moment itself, eventually giving rise to the impulse to memorialize it for generations.

But the initial proof of our commitment to perpetuate the memory of an event comes on the first anniversary. It is then when we have the opportunity to create the first link in the chain of memory.