Rishonim and the Rama in Shulchan Aruch, bring the minhag to say the hagada (from avodim hayeenu to lechaper…) on Shabbos HaGadol afternoon.
There are various reasons given for the minhag—they basically all have to do with the nissim which took place on this Shabbos, some say concerning the Korbon Pesach tied to their beds, some say concerning the battle of the bechorei Mitzrayim with the other Mitzrim.
Teshuvos Maharshal says it is to prepare for the seder night.
There are those who feel it should not be said, questioning the minhag. There are in turn many answers to these questions, upholding the minhag.
Rav Malinowitz wishes to make the point that it should not be laziness which governs whether we do or not—with the opposing tzad as an excuse. This was a normative minhag for most of K’lal Yisrael for many generations – it has its earliest source in Rishonim – do not discard lightly.