[1] 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.

However "certain men"[2] were ritually impure from contact with 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.

As elucidated in Mishnah Pesahim (9:1–4)[4] and its Gemara:[5] In net effect, anyone unavoidably prevented from offering the Korban Pesach on Passover may do so on Pesach Sheni provided that most of the nation were pure and brought on the first Passover.

The Rebbes of Nadvorna and related groups conduct a tish with four cups of wine, matza and maror, in the manner of a seder.