Aliya Summary: On the first anniversary of the Exodus, the Jews are instructed to bring the Paschal Offering. Certain individuals, however, couldn’t participate because they were ritually impure. These people lodged a complaint, which Moshe 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 Paschal Offering in its proper time must do so on the Second Passover. G‑d then informs Moshe the laws of the Second Passover.
This Pesach Sheni episode occurred before the counting of the People as recorded in the opening Parsha of Bamidbar. It was not placed at the beginning of the book because it is embarrassing to the People of Israel that they (we) only brought one Korban Pesach in the entire Wilderness period.