Pesachim 70A

לאו חובה היא. דאי סלקא דעתך חובה היא ⁠— תיתי בשבת, ותיתי במרובה, ותיתי בטומאה.

is not an obligation, meaning there is no Torah obligation to bring this offering. For if it should enter your mind to say that it is an obligation, it should come even on Shabbat, and it should come even when each member of the group will receive a large portion of the Paschal lamb, and it should come even in a state of ritual impurity.

ובמועט מיהו מאי טעמא אתיא? כדתניא: חגיגה הבאה עם הפסח נאכלת תחילה, כדי שיהא פסח נאכל על השבע.

The Talmud asks: If there is no obligation to bring this offering, what is the reason that it nevertheless comes when each person’s portion of the Paschal lamb is small? The Talmud explains that the reason is as it was taught in a baraita: The Festival peace-offering that comes with the Paschal lamb is eaten first; the reason for this is so that the Paschal lamb will be eaten when one is already satiated. The Paschal lamb should not be eaten in a needy manner, but rather in joy and when one is already filled to satisfaction.

ונאכלת לשני ימים וכו׳. מתניתין דלא כבן תימא. דתניא, בן תימא אומר: חגיגה הבאה עם הפסח הרי היא כפסח, ואינה נאכלת אלא ליום ולילה. וחגיגת חמשה עשר ⁠— נאכלת לשני ימים ולילה אחד.

The Mishnah taught that the Festival peace-offering of the 14th is eaten for two days and the intervening night. The Talmud notes that the Mishnah is not in accordance with the opinion of ben Teima, for it was taught in a baraita that ben Teima says: The Festival peace-offering that comes with the Paschal lamb on the 14th of Nisan is like the Paschal lamb and is eaten for only a day and a night, whereas the Festival peace-offering of the 15th, i.e., the Festival peace-offering brought on the 1st day of Passover, just as it is brought on the 1st day of each of the other Festivals, is treated like a regular peace-offering and is eaten for two days and one, i.e., the intervening, night.