A. Though we tend to regard the two names as interchangeable, historically there were two festivals – Pesach, 14 Nisan, when the paschal lamb was sacrificed on the day before the festival of Matzot (Ex. 12:6), and Chag HaMatzot, on the 15th, when the lamb was eaten al matzot um’rorim, “with unleavened bread and bitter herbs” (Ex. 12:8).
When the Temple was destroyed and the paschal sacrifice became impossible, the focus shifted to the matzah, though the name Pesach was retained, emphasising the element that was now lacking but would be restored in messianic times with the rebuilding of the Sanctuary.
The explanation Rabbi Levi Yitzchak of Berditchev gave for the two names was that the two partners, God and Israel, wanted to show their love for one another. He quoted Shir HaShirim 6:3, Ani l’dodi v’dodi li – “I am my Beloved’s and my Beloved is mine”. Hence God praised Israel for the faith with which they went into the desert prepared to eat matzah instead of bread (the Zohar actually calls matzah “the food of faith”); Israel praised God for passing over their homes when He brought the tenth plague.
The partnership between God and Israel is found in many other aspects of Pesach. Ex. 12:42 calls the night of the Exodus Leil Shimmurim, “A night of watchfulness”. Who was it who was waiting and watchful? Both God and Israel: God was waiting for the right moment to bring about the Exodus; Israel were waiting for the opportunity of leaving slavery behind and moving into freedom.