The offering of the Korban Pesach was at the core of that celebration. However “certain men” were ritually impure from contact with a human corpses, and were therefore ineligible to participate in the Korban Pesach. Faced with the conflict of the requirement to participate in the Korban Pesach and their ineligibility due to impurity, they approached Moses and Aaron for instructions, which resulted in the communication of the law of Pesach Sheni.
Pesach Sheni (Hebrew: פסח שני, TODAY IS Second Passover) occurs every year on 14 Iyar. This is exactly one month after 14 Nisan, the day before Passover, which was the day prescribed for bringing the Korban Pesach (“Paschal offering”, i.e. Passover lamb) in anticipation of that holiday. As described in the source text for this mitzvah (Numbers 9:1-14), the Israelites were about to celebrate Passover one year after leaving Egypt.