In parshat Emor (Vayikra 23:4), the Torah introduces all of the festivals, designating them as days of "mikraei kodesh". Expecting to encounter the first of the moadim, we are instead initially surprised to be told of the korban Pesach that is brought on the afternoon of the fourteenth day of Nissan. Only in the next verse do we read of "Chag haMazot" which commences on the fifteenth of the month. Why does the Torah present the preparation of the korban Pesach, which seemingly takes place before the festival, as the first of the Torah's festivals?
The Chizkuni suggests that the period in which the Pesach is offered and eaten, from the afternoon of the fourteenth of Nissan through the evening of the fifteenth, is halachically designated as "Pesach", while " Chag haMazot", which briefly overlaps that first night, extends seven days. The Netziv, in his commentary on the Torah, suggests that the bringing of the Pesach constituted a moed in its own right. In his haggadah, the Netziv explains that our yearning to experience other "moadim", refers to the offering of the Pesach which has the status of a festival, as reflected by its inclusion in the list of moadim in parshat Emor.
This perspective is projected by the Gaon of Vilna, as well. The Rama ( Yoreh Deah 399:3) rules that while a mourner in the midst of the shiva period should continue adhering to the mourning prohibitions until late in the day before other festivals, he concludes his mourning period at noon before the onset of Pesach. The Gaon explains that this distinction is based upon the fact that the korban Pesach, which was prepared in the afternoon prior to Chag haMatzot constitutes a moed, Chag haPesach, as indicated by its inclusion in parshat Emor.
The status of this period as a moed resonates in other halachot. Rambam ( Hilchot Yom Tov 8:17-19) formulates the prohibition to engage in "melachah" during this period as comparable to a "chol haMoed" (See Pesachim 50a-b and Rashi, Tosafot, Baal haMaor, Ramban, and Ra'vad op.cit.). Rabbeinu Chananel ( Pesachim 98a) describes the hallel that accompanied the slaughtering of the korban Pesach as a yom tov hallel (see Pesachim 64a, 95a, 117a, and Griz al ha-Rambam, Hilchot Korban Pesach). Rambam ( Hilchot Chametz 1:8) rules according to R. Yehudah's view ( Pesachim 28b) that chametz eaten in the afternoon of the fourteenth is subject to a lav and malkot because it is the period of the Pesach offering. Perhaps this link between korban Pesach and the prohibition of chametz can be explained on the basis that Chag haPesach has commenced although chag haMatzot has yet to have begun.
Why should this sacrifice generate a quasi-festival in its own right? R. Gamliel (Pesachim 116b) established that there are three indispensable components in the haggadah-seder experience: Pesach, matzah, and maror. Yet, the Pesach appears to have a disproportionate impact upon the other two. The maror constitutes a biblical obligation only in conjunction with the Pesach. Many aspects of matzah are defined by the timing of the Pesach. The Netziv (Imrei Shefer- regarding different formulations of matzah's role in R. Gamliel's triad. See, also, Maharshah, Pesachim 116b) argues that in the ideal circumstances in which there is a korban Pesach, that theme dominates our focus during the seder, relegating matzah and maror to a supportive role. Furthermore, he notes in his introduction to the haggadah that many unique practices at the seder can be traced to the effort to sustain the centrality of the korban Pesach even in our post-churban era.
Indeed, the korban Pesach's singular character and its pivotal role in Judaism is attested to in other halachic contexts. It is inextricably linked to the only other mitzvat aseh whose neglect triggers a punishment of karet, i.e. brit milah. Chazal note that the phrase "bedamayich chayii" refers to the blood of milah and korban Pesach. Circumcision is a prerequisite for participation in the korban Pesach. Just as the milah is an indispensable component in Jewish commitment, so too korban Pesach is perceived as crucial to Jewish destiny. Chazal (see Mechilta cited by Rashi, Shemot 12:48) even found it necessary to preclude the notion that one who converts to Judaism any time during the year would have to bring a korban Pesach, just as a male convert would have to undergo circumcision in the process of joining Klal Yisrael!
The Torah consistently describes the korban Pesach as "Pesach la-Hashem", implying a special expression of devotion and recognition of Hashem's role. The parshah of Pesach Sheni (Bamidbar, chapter 9) records the urgency with which Klal Yisrael regarded korban Pesach as a spiritual opportunity to forge a relationship with Hashem. It was inconceivable to the nation that there would not be another opportunity to experience this korban. The requirement that this sacrifice be undertaken strictly lishmah ( Mishneh Zevachim 2a), that it alone follow rather than precede the afternoon tamid and other anomalies associated with this korban (its minui, its notar, its yotze etc.) attest to its sui generis character. The fact is that this korban is not developed in the Torah in the context of other sacrifices (Vayikra, Zav, Pinchas etc.), just as it is not sacrificed within the traditional time frame bracketed by the two temidim. Unlike other korbonot, there is no focus even on ritzui, no hint of the otherwise ubiquitous concept of "ishei reiach nichoach", (see Chatam Sofer, Netziv and other poskim regarding the possibility of its sacrifice in the aftermath of the churban ha-bayit), and no special portion for the kohanim. Korban Pesach is sometimes formulated as a korban tzibbur, though the mitzvah devolves upon each individual and is achieved by means of voluntary groupings. This korban defies classification because its role is unique.
The Noda B'Yehudah, in his commentary to the haggadah explains that R. Gamliel's three components correspond to three tenets of our faith. Pesach symbolizes Hashem's personal providence (hashgachah peratit). Hashem's special supervision and devotion to Klal Yisrael was manifested in the personal intervention on the night of the fifteenth of Nissan. In his view, matzah accents the nation's faith, and maror is associated with the concept of reward and punishment. But the fact is that Pesach also integrates Hashem's hashgachah on the night of the fifteenth with Klal Yisrael's remarkable faith initiative in following the command to publicly slaughter the symbol of Egyptian divinity on the afternoon of the fourteenth of Nissan. The significance of this remarkable interaction between Klal Yisrael and Hashem establishes the korban Pesach as a singular symbol and vehicle of avodat Hashem, and even generates a period of kedushat ha-zeman.
R. Levi Yitzhak of Bardichev posits that the Torah formulates the holiday as Chag haMatzot because Hashem chooses to credit Klal Yisrael's extraordinary commitment evidenced by their rush to follow his norm notwithstanding the uncertainty of their situation, while Klal Yisrael traditionally refers to the same holiday as Chag haPesah, underscoring Hashem's extraordinary devotion to Klal Yisrael reflected by His personal intervention on their behalf ("lo al yedei malach" etc.). In fact, both themes also find expression in the preparation and eating of the Pesach itself during the quasi-moed of Chag haPesah that actually commences in the afternoon of the fourteenth, as recorded in parshat Emor, preceding, overlapping, setting the tone for, and ultimately enhancing the entire experience of Chag haMatzot.