What is Pesach Sheni? A year after Yetzias Mitzrayim, Hashem instructed Bnei Yisroel to bring the Korbon Pesach on the afternoon of the fourteenth of Nissan, and to eat it that evening, roasted over the fire, together with Matzah and Morror, as they had done on the previous year just before they left Mitzrayim. There were, however, certain persons who had become ritually impure through contact with a dead body, and could not, therefore, prepare the Korbon Pesach on that day.
They approached Moshe and Aaron…and they said: ‘…Why should we be deprived, and not be able to present Hashems’ offering in its time, amongst the rest of Klal Yisroel?’” (Numbers 9). In response to their plea, Hashem established the 14th of Iyar as a “Second Pesach” for anyone who was unable to bring the offering on its appointed time in the previous month.
There is a Minhag to mark this day by eating Matzah. Chabad.