Rashi points out that this third aliyah begins by describing what takes place in the month of Nissan in the second year after leaving Egypt, while the book of Bamidbar (starting a few parshas earlier) started one month later. This shows once again that the Torah is not necessarily written in chronological order.
In this aliyah G-d tells Moshe about the holiday of Pesach and all its statutes and ordinances and the Jewish people do them and observe Pesach. However, certain men cannot participate because they are tamai- mais (impure because of contact with a dead body) and therefore cannot participate with everyone else in the festivities.
These men feel bad that they cannot participate, so Moshe asks G-d about the matter, and G-d says that whoever is tamai mais, or far away, "to you or to your generations", gets a second chance one month later to observe the Pesach. This is the holiday of Pesach sheni that we observe on the 14th of Iyar, (shortly before Lag B'omer), when we eat some our leftover matzo from Pesach.
The Chassidic point of these words describing the second Pesach is that "it is never too late", all is never lost, one can always correct past actions. Even if one is defiled, even if one is far away, and even if this is "to you", that is to say, by your own choice and desire, it is nonetheless possible to rectify matters if the desire to do so is sincere.
