The Mekhilta addresses a gap in the Torah's instructions about eating matzah during Passover. The verse states, "Seven days shall you eat matzot" (Exodus 12:18). This clearly establishes the obligation for the daytime. But what about the nights?
The seven days of Passover include seven nights as well. Yet "days" in Hebrew — yamim — can refer specifically to daytime hours, excluding the nighttime. If the Torah meant only the daylight periods, then eating matzah at the Passover seder on the first night would not actually be required by this verse.
The Mekhilta finds the answer in the continuation of the same passage: "until the twenty-first day of the month in the evening." The word "evening" is the key. By specifying that the obligation extends through the evening of the twenty-first day, the Torah signals that the nighttime hours are included in the matzah requirement throughout the entire festival period.
Without this phrase, one might have assumed that the obligation to eat matzah applied only during daylight. The addition of "in the evening" ensures that every night of Passover — from the first seder through the final evening — carries the same obligation as every day.
This kind of close reading, where a seemingly redundant phrase resolves a genuine legal ambiguity, is characteristic of the Mekhilta's method. The Torah's apparent verbosity is never accidental. Every additional word closes a loophole or opens a new legal reality.