This Shabbat is שבת הגדול. It’s not clear when it got that name, but it’s at least a millenium. תוספות on שבת פז,ב mention it, and give the classic reason brought in the שולחן ערוך:
Avudraham gives a nice variation on that reason:
שבת הגדול means “Shabbat of Adulting”.
But other reasons are given.
But, as יששכר יעקבסון points out in חזון המקרא, there’s a grammatical problem. “שבת” is feminine. The name should be שבת הגדולה. The מטה משה notes that many שבתות are commonly named for their special Torah or haftorah readings, like שבת זכור, שבת נחמו, שבת חזון and שבת שובה. This is no exception. We have a special haftorah that contains the pasuk:
This is “שבת ”הגדול because we read of “יום ה׳ הגדול”
That is the last pasuk of the haftorah, even though is the the penultimate pasuk of the perek, because we don’t want to end the haftorah on a negative note. The perek ends with:
For the popular song, we just omit the second half of the last pasuk. We don’t do that with haftorot, so we repeat the penultimate line which is a happier thought (same as we do for איכה, קוהלת and שבת ראש חודש).
But the other named שבתות are named for a significant word in the first pasuk of the haftorah. Why name this day after the last pasuk? I haven’t seen an answer, but I would propose that in fact, הנה אנכי שלח לכם את אליה הנביא is the first pasuk of the haftorah. The haftorah in reality is just those two psukim. But we can’t have a 2-pasuk haftorah:
And we can’t keep going because this is the end not only of ספר מלאכי, but of all ספרי נביאים. This is the last נבואה. So we start enough psukim early to make 21 psukim (that would be why we start in the middle of מלאכי's vision, with פסוק ד.
So why this haftorah on this Shabbat? There’s an obvious reason, the connection between the redemption from Egypt and the eschatological vision of the redemption of ימות משיח:
But this haftorah isn’t about the גאולה. There are lots of options in נביאים for eschatology. This haftorah is about אליהו as the harbinger of the גאולה. It’s about what happens before משיח.
We usually think of אליהו‘s role simply as announcing משיח’s coming, so we have כוס של אליהו as a symbol of the fifth לשון של גאולה, “וְהֵבֵאתִי אֶתְכֶם אֶל הָאָרֶץ”. Or as the one who will decide halachic questions, תשבי יתרץ קושיות ובעיות (so כוס של אליהו reflects the argument whether there should be a fifth cup or not, and אליהו will decide). Rabbi Shulman will be speaking on the question of the fifth cup at his דרשה לשבת הגדול later today, ומושך הרבה והעם אינן נפטרין לבתיהן עד שישמעו שיגמור את הכל.
But that’s not what the haftorah says: והשיב לב אבות על בנים ולב בנים על אבותם. And, whatever that means, it’s important. Because without it, פן אבוא והכיתי את הארץ חרם. The יום ה׳ הגדול והנורא will not be a day of גאולה, but of destruction.
What is השיב לב אבות על בנים ולב בנים על אבותם?
רבי יהושע understands השיב לב אבות על בנים ולב בנים על אבותם as a statement about יחוס.
רבי יהודה agrees that it is about יחוס but that אליהו wouldn’t reject anyone who has a חזקה of being Jewish.
רבי שמעון takes אבות and בנים metaphorically:
And the חכמים take אבות and בנים as referring to everyone, and take לב literally. And that is the message of שבת הגדול: If we can’t get along, then there will be no גאולה.
Note that the song is much more gentle than the נביא. In the song, Mashiach just leaves. The threat in מלאכי is worse: והכיתי את הארץ חרם.
Pesach is a great time to get together with family and argue about everything. We each have our own stringencies and won’t eat in each other’s houses. But the message of שבת הגדול is that while בניסן נגאלו and בניסן עתידין ליגאל, it is possible for all that גאולה to fall apart if we can’t get along.
But in the end, the נביא's message is one of hope. אליהו will come, and לב אבות and לב בנים will come together. That is what faith in ביאת משיח means.