This Shabbat is called שבת הגדול or some call it שבת הנס הגדול. The great miracle was that on that Shabbat in Egypt fell on the tenth of Nissan. The Jews took the Egyptian lambs that they worshipped as a deity, and tied it to their bedposts. The miracle was that the Egyptians said nothing when told that these animals would be offered as sacrifices.
The Haftarah from מלאכי also speaks of the יום הגדול, the great day when Mashiach will come. For these reasons, the Shabbat is called Shabbat Hagadol.
It is customary to read part of the Haggadah on Shabbat afternoon to get ready for the Seder.
Rabbis give Shabbat Hagadol Drashot in order to review Halachot and arouse the nation to get closer to Hashem.