107:3 On Shabbos HaGadol (the Shabbos before Pesach) at mincha, the practice is not to recite Barchi Nafshi (Psalm 104). Instead, we recite Avadim Hayinu (“we were slaves…,” from the Haggadah) because Shabbos HaGadol was the start of our redemption from Egypt and the accompanying miracles. (The Gra disagrees with the custom because of the paragraph in the Haggadah that says, “I might think while it is yet day…” – Bi’ur Halacha 430:1 s.v. b’mincha.)
108:1 It is written (Exodus 12:17) “You shall watch the matzos,” from which we learn to watch the wheat intended for the mitvah of matzah to keep water from getting on it. According to some of the Sages, this guarding must begin immediately after harvesting, but the accepted practice is that it is sufficient to watch the wheat from the time it is brought to the mill. Some, however, are scrupulous in this mitzvah and watch from the time of harvesting; this is a proper way to act. It is appropriate to be careful and not to stand the stalks tied together until they have completely dried and turned white. Otherwise, if rain falls on them, they will leaven, even if they are tied together, because they no longer need the ground. It is therefore proper to harvest the wheat while it is still a little green. It is good if one arranges matzah for himself for the entire holiday from wheat that was watched from the time of harvesting. If not, it is still appropriate for him to use such matzah for the two nights of the Seder.