The Month of Nissan
3. On Shabbos HaGodol (lit: "the Great Shabbos," referring to the Shabbos
before Pesach) (1) during Mincha, it is customary not to read the
Psalm "Borchi Nafshi",but rather to read [the Haggadah from the
words] "Avodim Hayinu" [until the conclusion of the passage beginning "Al
achas kamoh v'chamoh,"] (2) because the redemption [from Egypt] and the
miracles (3) began on Shabbos HaGodol.
(1) God commanded the Israelites in Egypt to designate, on the tenth of
Nissan (five days before the Exodus), a male sheep for the Pesach offering
(which was to be slaughtered on the afternoon of the 14th and eaten that
night, the night of the Exodus). They tied the animal to their bedposts,
and although the Egyptians worshipped the ram as a god and knew what the
Jews were going to do with the animal, they miraculously did not try to
stop them. That year, the 10th of Nissan was a Shabbos, and in
commemoration of that miracle, we call the Shabbos before Pesach, "Shabbos
HaGodol" (Tur 430).
(2) The Vilna Gaon did not agree with this custom and thus many people do
not follow it.
(3) That is, the miracles during the final week in Egypt.