Pesach Sheni

From Chabad.org:

A year after the Exodus, G‑d instructed the people of Israel to bring the Passover offering on the afternoon of the fourteenth of Nissan, and to eat it that evening, roasted over the fire, together with matzah and bitter herbs, as they had done the previous year just before they left Egypt. “There were, however, certain persons who had become ritually impure through contact with a dead body, and could not, therefore, prepare the Passover offering on that day. They approached Moses and Aaron . . . and they said: ‘. . . Why should we be deprived, and not be able to present G‑d’s offering in its time, amongst the children of Israel?’” (Numbers 9:6–7).

In response to their plea, G‑d established the 14th of Iyar as a “Second Passover” (Pesach Sheni) for anyone who was unable to bring the offering on its appointed time in the previous month. The day thus represents the “second chance” achieved by teshuvah, the power of repentance and “return.” In the words of Rabbi Yosef Yitzchak of Lubavitch, “The Second Passover means that it’s never a ‘lost case.’”

It is customary to mark this day by eating matzah, shmurah matzah if possible.

And skipping tachanun. Now go eat some shmurah matzah! Also, follow the link to Chabad.org for various elaborations on the homiletic message of Pesach Sheni: "There is always a second chance."