On the fifth day of Passover, Jews read the Torah portion that instructs Moses to establish Pesach Sheni (פסח שני), literally meaning "Second Passover." Pesach Sheni was established as a second opportunity for those who were unable to bring the Passover sacrifice on the first Passover due to being impure, being too far from the Temple, or being unable to participate for some other reason. It allowed them to offer the Pesach sacrifice exactly one month after the first day of Passover, on the 14th day of the month, Iyar.
Today, Pesach Sheni is not widely observed as the Temple in Jerusalem no longer stands, and the Pesach sacrifice is no longer offered. However, some Jews still commemorate the day by reciting special prayers and eating matzah.