The Admor of Kretshenif Yerushalayim conducting a seder on Pesach Sheni in the Kretshenif Beit Midrash in Jerusalem, Israel; 2005. x
Pesach Sheni, or Second Passover, occurs on the 14th of the Hebrew month of Iyar. The holiday comes from the Torah in the Book of Numbers Chapter 9 where Moses announces that the Passover sacrifice (Korban Pesach) can only be eaten on a specific date and only by those who are ritually pure. This angers people in the Israelite nation who had come in contact with the dead and were now considered impure and unable to bring the offering and thus G-d responds by declaring that anyone who is unable to bring the sacrifice – either due to ritual impurity or an inability to reach Jerusalem (the site of offering) during Passover – can instead make the sacrifice on the 14th of Iyar, a full month after Passover.  The bringing of the Passover offering was considered so important that this is the only instance of a Torah commandment in which an official "make-up day" is established.
Nowadays, Passover Sheni is celebrated only symbolically, as the practice of bringing sacrifices has been discontinued since the destruction of the Second Temple. Instead, it is now customary to eat a piece of matzah on the holiday while tachanun is omitted from the prayer service to honor the joyous occasion.
Pesach Sheni begins in 2017 on the evening of May 9th and continues until the evening of May 10th.
