Make-up Date
There is a principle regarding the offering of sacrifices. If the designated time for the obligation of a specific sacrifice, passes, the Korban is nullified. This is called, עבר זמנו בטל קרבנו, “the time passes, sacrifice is nullified.”
The exception to this rule is that there is a make-up date for the Korban Pesach. If someone was in a state of impurity, or was unable to get to the Beit Hamikdash due to unforeseen circumstances, he can offer the sacrifice one month later.
The fourteenth of Iyar is known as Pesach Sheini. The lamb is offered on the altar and is followed by a Seder like meal. The menu consisted of Matza and bitter herbs.
We see the particular importance given to this sacrifice, more than any other sacrifice. There are only two positive Mitzvot in the Torah, whose non-fulfillment comes with the punishment of Karet. (The other positive commandment is Brit Mila.) All of the other 246 positive commandments, do not carry any punishment at all if they are not fulfilled.
The Korban Pesach is special because it represents a strong identification with the Jewish people. Its offering is an affirmation that the events in Egypt actually happened. They are not fable but showed how Hashem openly revealed Himself when we began as a people. It affirms the Divine aspect of our religion. Everything comes from Sinai and all of our observances are part of the Oral and Wriiten Law, come from there.
This is why this particular sacrifice has a make-up date if the first date was missed. Every Jew needed to make this affirmation that he was proud to be part of Am Yisrael.