The Shabbos before Pesach is referred to as Shabbos Hagadol, the great Shabbos. Two of the reasons for this name are as follows.

On this Shabbos the Jewish people in Egypt were commanded to take a lamb for the Pesach sacrifice, and guard it until the eve of Passover when it could be sacrificed. Being that the lamb was an idol of the Egyptians, it was a miracle the Egyptians didn't rise and attack the Jews.

Another reason is based on the Haftorah we recite this Shabbos. The prophet talks about the Great Day when the Messiah will come. Before that day, Elijah the Prophet will blow shofar to announce the Messiah's coming, and the true and final redemption will begin.

Orach Chaim of the Rama 430:1 states a custom to read the Passover Hagada on Shabbos afternoon from the "avadim hayeenu" (we were slaves in Egypt) until "lechaper al col avonoseinu" (To atone for all our sins).

The Vilna Gaon questions this custom. The Mechilta, which is in the actual text of the Hagada, says "One might think that the obligation to discuss the Exodus began with Rosh Chodesh Nissan. Therefore the Torah states, 'You shall tell your son on that day.' The term 'on that day' might be understood to mean only during the daytime. Therefore the Torah adds: 'It is because of this that G-d did THIS for me when I went out of Egypt.' The word THIS implies something tangible. Therefore, 'You shall tell your son' applies only when Matzah and Maror are placed before you" The Vilna Gaon says that the Mechilta implies that we are precluded from reciting the Hagada before the night of the seder.

In practice then, there are only some people that follow the Rama, reciting the Hagada on Shabbos Hagadol, the beginning of the miracles, and also to familiarize themselves with the text which was not recited since the previous year.

In the Haftorah, taken from the end of the Book of Malachi, the announcement of the future redemption reminds us that the Redemption from Egypt began on the Shabbos before.

Another significance of the Haftorah is that in predicting the final redemption, Malachi at the same time submits a warning to be careful with tithes "But if the people give their tithes, they are blessed as it says, 'Bring all the tithes into the storehouse that there may be meat in My house, and prove Me now with this, says G-d, if I will not open for you the windows of the heavens and pour out for you a blessing without end.'" This is a warning for people to be scrupulous with tithing crops so that they will not be punished for its neglect at the time of the final redemption.

Adapted from R. Blumenkrantz's The Laws of Passover - A Digest