How do we celebrate Chag HaPesach? Let the sources speak:
E: There is one day of Pesach. A sheep is to be slaughtered and eaten, and leavened bread is not eaten, because we left Mitzraim in haste. This day is not a Chag in the technical sense.
J: There are seven days of Chag HaMatzot. Unleavened bread is eaten as a reminder of the Manna that fell in the Desert. The seventh day is a Chag to HaShem.
D: There are seven days of Pesach/Chag HaMatzot. The first day, the Pesach sacrifice is brought, in the place that HaShem will choose (the Temple in Yerushalayim). It is eaten in the night of the first day. There are daily sacrifices for each of the seven days. Unleavened bread is eaten, to commemorate that we left Mitzraim in haste. The seventh day is a day of convocation, and we do not work. (Devarim 16:1-8)
P: There are seven days of Pesach/Chag HaMatzot. The first and the seventh day are days of holy convocation, and we do not work. The Pesach to HaShem is eaten in the night of the first day. We eat unleavened bread for seven days, because it is Chag Matzot. There are daily sacrifices for each of the seven days. (Vayikra 23:4-8, Shemot 12:1-11)
D seems to be the first to combine Pesach and Chag HaMatzot, and the first to prescribe daily sacrifices in the Temple. Only P prescribes that there is a chag on the first day, the chag of HaShem's Pesach proper. Regarding Chag HaMatzot, the Halacha is that Chag HaMatzot is according to J.
