Why is the Sabbath before Pesach known as 'Shabbos HaGadol' or the Great Shabbos?
The simplest answer comes from the reading of the Haftorah, after the Torah reading, which comes from Malachi 3 which concludes with, "Behold I will send you Elijah the prophet before the coming of the great and awesome day of the Lord." Or, in Hebrew, "HaGadol VeHanora".
What does the reading from Malachi, which involves a reproof of the Jewish people and a promised final redemption have to do with Pesach? But then why does Elijah the Prophet show up at the Seder?
The answer once again is in Malachi.
"I will send you Elijah the prophet before the coming of the great and awesome day of the Lord, that he may turn the heart of the fathers back through the children, and the heart of the children back through their fathers-lest I come and smite the earth with utter destruction."
There are two times that Elijah is set to appear, the Bris, the circumcision, and the Passover Seder.
What do both of them have in common? They're part of the transmission of the tradition from fathers to sons. The Bris is the physical transmission of the covenant while the Seder is a verbal transmission.
This ties together the role of the Seder, not just as a memorialization of the past, but a call to the future. On Passover, we were again, and on Passover we are destined to be saved again. Elijah's reunification of generations is a prerequisite for a more positive salvation. The Malachi reading begins with a reproof of the Jews who have become cynical, who ask, like the wicked son of the Seder, what is the point of serving G-d?
"It is futile to serve God, and what do we gain from keeping His commandments and for going about in anxious worry because of the Lord of Hosts? And now we praise wicked men. Those who do evil are built up, they challenge G-d, they have, nevertheless, escaped."
This cynicism leads to the other enumerated sins. The people no longer have faith in G-d and do not pay tithes. They practice witchcraft, false oaths and oppress the weak. These are all sins that are closely associated with a disregard and a lack of fear of G-d.
The Jews have lost faith. And they lost that faith because of the generation gap that Elijah is sent to remedy.
The cynicism that leads to a loss of faith comes naturally with life experience. As time passes, disappointments accumulate, idealism no longer seems to pay off and anecdotal evidence seems to suggest that the wicked prosper because they do what they want while everyone else misses out.
When a generation gap emerges, the adults with life experience have become cynical in this fashion while the younger idealistic generation rejects their cynicism and their wisdom and burns down everything that was in pursuit of a new ideal. As that younger generation ages, they in turn become cynical, seeing their peers 'sell out' and the cycle repeats itself.
Malachi is arguably telling us that the chain of transmission, the mesorah, depends on uniting the wisdom of the older generation with the idealism of the younger generation. It is the young who are best able to refresh the inherited ideals with their idealism and the older generation that can feel revived, even in the face of the cynicism of their accumulated life experience, with the idealism of the young.
Working together, the old and the young can restore faith and build a better world, set apart from each other, the old corrupt and the young wreak destruction.
The Seder, when everyone gathers together, is the ideal moment for the revival of faith through a dialogue of generations. It is a time when everyone is meant to listen and to speak, when we can all learn from each other, and the hearts of the fathers and sons can come together again.
Pesach is the ultimate family holiday because redemption requires the unity of Jews as a family.
When the Prophet Elijah comes to see to his cup, late in the seder, it is hoped that this has been accomplished and the pathway to the redemption has been paved by the discussions that have come before leading to a family and a nation unified in its mission by both wisdom and ideals.