I have seen a custom (it seems to me primarily among Chassidim) to eat matza on 14 Iyar (known as Pesach Sheni). What is the earliest source for this custom? What are the reasons given for following it?
As an aside, I always found this custom odd, as the matza that was eaten with the Korban Pesach Sheni was eaten on the night of 15 Iyar. 14 Iyar is the day on whose afternoon the Korban was offered. Shouldn't the custom be to eat matza on 15 Iyar?
Very good question. The Piskei Tshuvos 5:492 brings down that scrupulous individuals are accustomed to eat matzah on peasach sheni. In footnote 9, he brings down that in the siddur Yaavetz (Rav Yaakov Emden) that it was revealed to him from the heavens that the kedusha of pesach and matzah lasts until pesach sheni because when they went out from Egypt they ate the matzos that they came out with until the 15 of iyar (whereupon the man fell as it says in Kiddushin 38).
Regarding why not the 15 of Iyar, he brings down in footnote 10 that some people do eat it at night on the 15th just like the regular pesach. The are even some who eat it with maror and a cooked egg (per footnote 11).
The Lubavitcher Rebbe (Igress Kodesh vol. 2 pg. 353) quotes the Darkei Chaim VeShalom of Munkatch, that although the eating of the Korban Pesach took place on the night of the 15th of Iyar, since they would offer it on the 14th we customarily eat Matzah then - as the beginning is considered the main thing. (However, the Munkatcher would be careful to eat Matza in the night as well, and Rabbi Yehuda Leib Groner (the Rebbe's personal secretary) reported that most years the Rebbe would too.)
