From On the first anniversary of the Exodus, the Jews are instructed to bring the Pesach Offering. Certain individuals, however, couldn’t participate because they were ritually impure. These people lodged a complaint, which Moses then transmitted to G‑d. G‑d responds by designating a “Second Passover” to be observed exactly one month later. Anyone who could not offer the Pesach Offering in its proper time must do so on the Second Passover. G‑d then informs Moses the laws of the Second Passover.

Typically the Torah introduces things by saying “G-d spoke to Moshe to relay the following…”. However, in this instance the Torah relates that some people came to “complain” that they were unclean and couldn’t offer the Pesach offering, and didn’t want to be excluded. So Moshe asked G-d what to do, and G-d implemented the Second Pesach. The Sifri explains that it was in the merit of those who felt cheated that they were mentioned in the Torah, because they felt neglected for not being able to perform a single Mitzvah. Obviously great desire to emulate.