The Torah states that a Hebrew bondsman "shall go out free" on "the seventh" year. But the seventh year of what? The Mekhilta identified two possible readings and used a careful textual argument to choose between them.
The seventh year could mean the seventh year from the date of sale — the bondsman's personal clock. Or it could mean the seventh calendar year — the shemitah, the sabbatical year that applies to the entire nation. The distinction matters enormously. Under the first reading, each bondsman has his own release date. Under the second, all bondsmen are released simultaneously in the sabbatical year, regardless of when they were sold.
The Mekhilta resolves this by pointing to the full verse: "Six years shall he serve, and on the seventh he shall go out free." The "six years" of service is counted from the moment of his sale. He serves six full years from his personal start date, and on the seventh, he goes free. It is the seventh year of the sale, not the seventh year of the national calendar.
This ruling ensured that every bondsman received a full six years of labor — no more and no less — before his release. If the sabbatical year had been the trigger, some bondsmen would serve nearly seven years while others served barely one, depending on when they happened to be sold. The personal clock created fairness. Each person's servitude was measured by his own timeline.