“On the second month on the fourteenth day in the afternoon they shall do it!” (Bamidbar 9:11)
It is a positive commandment that anyone who was unable to bring the first pascal offering on the fourteenth of Nissan, (e.g. if he was impure or far away) must bring a substitute pascal offering on the fourteenth of Iyar, as it says “On the second month on the fourteenth day in the afternoon they shall do it!”. Chazal taught us that anyone who did not bring the first offering either because he was unable or by mistake, or even on purpose, can and must bring the second.
Amongst the roots of this commandment is that the commandment of Passover is a strong and clear sign to all the seers of the sun that Hashem created the world, for at that time Hashem performed great and wondrous miracles, and he changed the nature of the world in front of many nations, and all of the people of the world saw his supervision and ability in this world, and at that time everyone believed in that clear truth that He created the world from nothing at the time that He wanted. This was proven in that even though naturally one cannot create something from nothing, but so too it is impossible to naturally split the depths of the sea for a large number of people to cross over on dry land and return to their place, and it is impossible for a large nation to survive in the dessert for forty years on bread which fell from the sky every day, and so too al of the other signs and wonders that He performed at that time which were all contrary to the laws of nature. The creation of the world is the strong pillar in our belief and Torah, and this is obvious. It was therefore the will of Hashem that every member of Klal Yisrael should merit to perform this commandment, and that no accident should stop the performance of this commandment, for if he was unable tom perform it at the first opportunity, he can do it the next month. Since this is a great foundation of our belief, this obligation applies even to a person who converted between the two Passovers, and a child who grew up between the two.
Amongst the laws of this commandment are that on the second pascal offering, he may have both leavened bread and unleavened bread in the house. It only applies for one day, and there is no festival or prohibitions of work, nor do we say Hallel when eating it. One does, however, say Hallel when offering it up, and it is eaten roasted with unleavened bread and bitter vegetables, overrides Shabbos, and must not be left over until the morning nor must a bone in it be broken.
This commandment applies when the temple was standing, to men as an obligation and to women as an option.