On 14 Nisan, G-d commanded the people to observe Passover. However, some were ritually defiled and unable to participate. They complained about being left out, and in response, G-d informed Moses that whoever was unable to perform the Passover rituals on time should do so a month later, on 14 Iyar.
Numbers 9:1–14
Passover requires preparation, abstinence from leaven, and lasts for a week. The second Passover, in contrast, requires almost no preparation, no prohibition of leaven, and lasts for one day.
This is because at Passover – our annual rebirth as a people and as individuals – we begin life anew, with a clean slate. As such, we can follow the path of righteousness, ascending the ladder of spiritual growth by refraining from all evil and doing only good.
The second Passover, in contrast, is the path of repentance. Repentance occurs as a sudden inspiration, and therefore does not entail extensive preparation. Repentance is a humbling admission of fault, and therefore leaven and the inflated egocentricity it represents are not threats. Finally, repentance is a radical departure from our previous, flawed behavior, which occurs in an instant and is therefore celebrated for only one day.
In the dynamics of spiritual life, we are sometimes on the path of righteousness and sometimes on the path of repentance, and should therefore become expert at following both.
--Daily Wisdom Volume 3