(Exodus 13:8) commands, "And you shall tell your son on that day." But when exactly is "that day"? The verse sits within a passage about the month of Nisan, so one might think the obligation to tell the story of the Exodus begins on the first day of the month — Rosh Chodesh Nisan, two full weeks before the Seder.
The Mekhilta rejects this. The verse specifies "on that day" — not "in that month." So the telling is restricted to a particular day.
But a second problem arises. If the obligation is "on that day," perhaps it means during the daytime hours of the fourteenth or fifteenth of Nisan — while the sun is still up, before the Seder night begins? After all, "day" typically means daytime in the Torah.
The Mekhilta eliminates this reading as well by pointing to the continuation of the verse: "for the sake of this." The word "this" is a demonstrative — it points at something present and visible. What is present? The matzah and maror, the unleavened bread and bitter herbs that sit on the table during the Seder night, on the fifteenth of Nisan.
The obligation to tell the story therefore applies specifically on the night when matzah and maror are resting in front of you. Not two weeks early. Not during the afternoon. On Seder night, with the physical symbols of slavery and redemption laid out on the table, a parent turns to a child and tells the story. The timing is not arbitrary — it is theatrical. The props must be present for the telling to begin.