The verse (Exodus 13:7) commands, "Matzoth shall be eaten the seven days, and chametz shall not be seen unto you." A straightforward reading suggests these two rules — eat matzah, avoid chametz — operate on the same timeline. But the rabbis of the Mekhilta identified a discrepancy that required resolution.
From other sources, the sages had already established that chametz becomes forbidden starting from the sixth hour of the fourteenth day of Nisan — that is, midday on the day before the Seder night. The prohibition kicks in several hours before the holiday officially begins. So one might logically assume that the obligation to eat matzah also starts from that same moment — from midday on the fourteenth.
The Mekhilta rejects this assumption. The verse links the two commandments — eating matzah and not seeing chametz — within the framework of "the seven days." Both rules apply during the seven days of the festival proper, not before. The earlier prohibition on chametz from the sixth hour is derived from a separate source and operates independently.
This distinction matters for practical observance. While chametz must be cleared away by midday on the fourteenth of Nisan, the positive commandment to eat matzah does not begin until the night of the fifteenth — the Seder night itself. The two obligations share a holiday but not an identical start time. The Torah treats the removal of the old and the embrace of the new as related but distinct acts.