Pesach Sheni/The Second Passover
On the first anniversary of the exodus from Egypt, the Children of Israel prepared to celebrate their first Passover as free people. God decreed that they should eat matzah and maror (bitter herbs) in commemoration of the great event, and, most importantly, that the Israelites should all partake of the Passover sacrifice (lamb).
On the eve of the second Passover, Moses was approached by a group of distraught men. “We are unclean because of the dead body of a man; why are we being held back so that we cannot bring the offering of God in its appointed time among the children of Israel?” (Numbers 9:7)
Contact with the dead rendered a person tamei, spiritually impure, and any person who was tamei was forbidden to partake of the Passover lamb.
In response to their plea, Moses sought instruction from God. God responded that anyone who was tamei due to contact with death or who was on a distant journey at the time of the Passover offering (14th of Nisan), was then obligated to offer the Passover lamb one month later, on the 14th of Iyar. Those celebrating Pesach Sheni (the Second Passover) must eat the meat of the sacrifice together with matzah and maror, exactly as on a regular Passover.
Today, without a Temple, no one is able to bring a Passover sacrifice and everyone is in some state of tumah (ritual impurity). Thus the laws of Pesach Sheni have little practical effect in day to day Jewish life. However, there is a custom to eat some matzah on the 14th of Iyar to mark the date of Pesach Sheni for ourselves and for future generations.