The Torah teaches (see Bamidbar 9:10-14) that someone who was unable to sacrifice the korban Pesah at the proper time because he was ritually defiled or because he was far from Jerusalem, is obligated to come to the Temple one month later, on the 14th of Iyyar and bring a Pesah sheni – a “second Pesah.” One of the basic questions associated with this sacrifice is whether it is merely a replacement for the first, or if Pesah sheni is a separate holiday, albeit one that is only obligatory on those people who did not succeed in bringing the sacrifice the first time.
The Mishna on our daf teaches that for all that the Torah commands that the same rules apply to the Pesah sheni that applied to the first Pesah, nevertheless there are significant differences between the two. For example, the commandment to rid oneself of hametz before the sacrifice is brought only applies on the regular Pesah, and not on Pesah sheni. Similarly, Hallel is recited while eating the sacrifice on Pesah rishon (first), but not on Pesah sheni. The Mishna mentions other laws that apply to both, like the recitation of Hallel while the korban is being sacrificed, that the meat is eaten roasted together with matza and marror, and that both “push aside” Shabbat should the day that the sacrifice needs to be brought fall on Shabbat.
Tosafot point out that the Mishna is only giving examples, and that there are other laws that are unique to Pesah rishon. As a case in point, the Jerusalem Talmud notes that the korban Pesah is accompanied by a korban hagiga (see Pesahim daf 70) only on Pesah rishon and not on Pesah sheni.
The Gemara asks: What is the reason that hallel must be recited while one prepares the Paschal lamb on the second Pesah? The Gemara answers…if you wish, say that this halakha simply makes logical sense: Is it possible that the Jewish people are slaughtering their Paschal lambs or taking their lulavim on Sukkot and not reciting hallel? It is inconceivable that they would not be reciting hallel and there is no need for an explicit biblical source for this halakha.
This argument, which can be applied to every one of the Jewish holidays, indicates that the tradition of reciting Hallel is an ancient one. Nevertheless, once we establish the centrality of the recitation of Hallel to the celebration of the holidays, why is it not said while the korban Pesah is eaten on Pesah sheni? One answer that is suggested points to the fact that Hallel is usually recited only during the day, and we need a special pasuk to introduce the idea of reciting it at night on Pesah. The passage brought by the Gemara to suggest saying Hallel at night appears in Yeshayahu (30:29) “the song should be for you as the night of the celebration of the holiday” which is understood to teach that a song – the Hallel – is appropriate only when there is a holiday being celebrated. For all the importance of Pesah sheni, it is not a Yom Tov, as work is permitted, etc.