This Shabbat is called Shabbat Hagadol. It is considered an extremely important Shabbat. Aside from the custom of reading part of the Hagaddah during the course of the day, it is meant to be a Shabbat of a spiritual nature.
Just like Shabbat Shuva before Yom Kippur, on these two Shabbatot we are to listen to Drashot of big rabbis to prepare us for the Pesach holiday. Because of all of the cleaning and physical preparations, we easily can forget the spiritual message of Pesach.
The removal of Chametz symbolizes the removal of haughtiness and negativity. The eating of Matza is to emphasize humility in our service of Hashem.
There are two reasons why we call this Shabbat, שבת הגדול. The first is that the Haftarah uses the words, הנה היום הגדול בא, that a great day is coming. And the second reason is that some call it שבת הנס הגדול, the Shabbat of the great miracle. This refers to the tying up of the lambs, an object of Egyptian idol worship, to the bedposts on the Shabbat before the Exodus. These lambs were offered as the Pesach sacrifice and the Egyptians did not protest.
This Shabbat should be taken very seriously so that we appreciate the overall specialness of the holiday of Pesach. Shabbat Shalom