Pesach Sheni occurs on the 14th of Iyar. The holiday is mentioned in the Torah in Numbers 9. Moses announces that the Passover sacrifice (Korban Pesach, or Passover lamb) may only be eaten by people who are ritually pure. Men come to Moses, complaining that as people who have come into contact with the dead, and therefore ritually unclean, they are unable to fulfill the mitzvah of Passover. Moses consults God who responds by announcing that anyone who is unable to sacrifice the paschal lamb on the 14th of Nisan, either due to defilement or inability to journey to the place of sacrifice in time, is to perform the sacrifice on the 14th of Iyar, a full month later, and eat the paschal lamb along with matzah and maror.

Today, after the destruction of the Temple in Jerusalem and the absence of the red heifer, Jews are unable to perform the Passover sacrifice, neither on Passover nor on Pesach Sheni. It is customary to eat a piece of Matzah. Tachanun and other penitential prayers are also omitted due to the festive nature of the day.