Or HaChaim on Numbers 9:1/אור החיים על במדבר ט׳:א׳

1 בשנה השנית, in the second year, etc. Why did the Torah change its style in this instance by mentioning first the year and then the month of this event? When you look at the beginning of the Book of Numbers you will find that the Torah first mentions the month before mentioning the year. Not only this, but in our verse here the Torah does not even mention the month at all until after it referred to the Exodus. It would not have been necessary to interrupt mention of the date if the month in which G'd issued this command had been mentioned first. Another peculiarity in this paragraph is the absence of the customary instruction to Moses to "speak to the children of Israel!" The letter ו in the word ויעשו also needs explaining. To which previous instruction does that letter ו refer? Why was it altogether necessary to issue instructions to the Israelites to observe the Passover when this had already been commanded in Exodus 12,43? Apparently, the Torah was afraid that the Israelites had understood the previous commandment to apply only in Egypt and in the land of Israel so that the Torah had to make plain that it was a commandment which applied also during their stay in the desert.

2It appears that the repetition has to do with the prohibition of a בן נכר to eat of the Passover. According to the Mechilta on Exodus 12,43 that expression meant that anyone who was either a Gentile or a Jew who deliberately flouted G'd's laws was "estranged" and was not allowed to eat of that sacrifice. Seeing that in the interval since the Exodus the Israelites had become guilty of making the golden calf and worshiping it they had reason to believe that the injunction in Exodus 12,43 applied to them. This is especially so seeing that we are taught in Chulin 5 that anyone who acknowledges any deity other than G'd is considered as if he had denied all of Judaism. While it is true that G'd had accepted the repentance of the Jewish people after Moses had stayed on Mount Sinai three times for 40 days to beg for forgiveness, the people had not yet been told that that sin had been atoned for and there remained a lingering doubt concerning that forgiveness. The people therefore needed special permission in order to proceed and to offer the Passover on the anniversary of the Exodus. This is also why the Torah proceeds without the customary "speak to the children of Israel" to introduce this command simply as a corollary to the original Passover, and it uses the conjunctive letter ו when writing "and they shall perform the Passover." The word ויעשו tells the Israelites that not only had G'd reconsidered His original intent of destroying them but He had even consented to let them bring the Passover.

3Now we can also better understand why the Torah began this chapter by first mentioning the year in which this command was issued. Seeing that the whole paragraph only intended to sanction the fact that the Israelites would offer the Passover, the Torah speaks of the second year so that we would appreciate that this was after the episode of the golden calf. Had the Torah commenced by first mentioning the month we would not have been struck by the fact that the command was issued after the episode of the golden calf. G'd did not tell Moses: "speak to the children of Israel," as He did not want to command them to do this; rather He wanted to permit them to offer the Passover. Seeing that the Talmud (Pessachim 6) claims that this communication occurred on the first of Nissan, it may have occurred before the שכינה descended onto the Tabernacle on that day, and it was G'd's way of informing the people that they had been found worthy to offer the Passover. Subsequent to this communication the Presence of G'd descended and this is why we find the Torah continue in verse 15 with: וביום הקים את המשכן כסה הענן.