It says in Parashath Beha'alothkha that those who were unable to bring the Qorban Pesah (Paschal sacrifice), because they were Teme-eh Meth (ritually impure), or were travelling away from Yerushalayim (Jerusalem) at the time that the Qorban was required to be made, could bring it a month later. It was supposed to be brought on the fourteenth of Nissan, and those who were unable to do it then could bring it on the fourteenth of Iyyar. However, specifically excluded were those who were in the position of bringing it at the correct time and didn't bother.
Many of the laws for Pesah Sheni, (the second Pesah, as it is called), and specifically the Qorban Pesah were very similar on both occasions. The Qorban Pesah, on Pesah Sheni, also had to be eaten with Massah and Maror, and several other points were also the same. However, there was no requirement of not having Hames (leaven).
In commemoration of this, there are those who eat Massah on Pesah Sheni. The Ben Ish Hai says in Orah Hayyim, that the fourteenth of Iyyar, Pesah Sheni, is a holy day, where we should somewhat increase our rejoicing.
(See Orah Hayyim Haggadah, 'Inyan Hol Hamoed, Oth Lamad Aleph)
