Why is it called Shabbat HaGadol?
- Some explain that it's called Shabbat HaGadol because that was the Shabbat when a miracle occurred for the Jews in Egypt that the Egyptians didn't say anything when the Jews took sheep, which was the Egyptian god, in order to slaughter them for Pesach. 
- There are a number of Minhagim about which Haftara to read when Erev Pesach is on Shabbat HaGadol, and each community should follow its own custom.
Drasha of the Rabbi
- The minhag is for the Rabbi of the community to give a Drasha on Shabbat HaGadol on Hilchot Pesach.  However, if Shabbat HaGadol falls out on Erev Pesach, the Drasha should take place a week earlier. 
- If the Rabbi who usually gives the Drasha on Shabbat HaGadol is in mourning and there’s no other Rabbi to make a Drasha, it’s permissible to give the Drasha. 
Greetings on Shabbat Hagadol
Reading the Haggadah in advance
- On Shabbat HaGadol, the minhag is to read the Haggadah from Avadim Hayinu until LeChaper Al Avonotanu. 
- Shulchan Aruch 430:1, Mishna Brurah 430:1
- Yechave Daat 1:3
- Mishna Brurah 429:2, Chazon Ovadia page 32
- Mishna Brurah 430:1, Chazon Ovadia page 32
- Halichot Shlomo (vol 1, 2:7)
- Chazon Ovadia page 29
- Rama 430:1 based on Ravyah 225 writes that the minhag is to read the Haggadah from Avadim Hayinu until LeChaper Al Avonotanu on Shabbat HaGadol. This is quoted by Kitzur Shulchan Aruch 107:3. However, Beiur Halacha s.v. BeMincha quotes the Gra (Maaseh Rav #173) who didn't have the minhag to read the Hagadah on Shabbat.