Kommentar zu Schemot 12:18

עד יום האחד ועשרים UNTIL THE TWENTY-FIRST DAY — Why is this stated? Has it not already been stated, (v. 15) “Seven days [shall ye eat unleavened bread]” (beginning on the fifteenth day, as Rashi shows in his comment on v. 14 and therefore terminating on the twenty-first)? The answer is: Since it is said in that verse, “[seven] days [shalt thou eat unleavened bread]”, we may ask whence do we derive that the prohibition of eating unleavened bread extends also to the nights of these seven days? Scripture therefore states here, “[Ye shall eat unleavened bread] until the twenty-first-day in the evening” (Mekhilta d'Rabbi Yishmael 12:18).

Why is this said. . . You might ask: Does this verse not imply that eating matzoh is obligatory for all seven days, yet before, Rashi clearly explained that for all seven days it is optional? The answer is: Rashi indeed said before that eating matzoh is optional, as that was the verse’s subject. But here, the subject is the removal of leaven from the home, as is seen in the following verse, “For seven days no leaven may be found in your homes.” And when Rashi mentioned “seven days,” he was not referring to: “You must eat matzos for seven days” (v. 15), rather he was referring to: “For seven days no leaven may be found” (v. 19). And if it said only, “Seven days no leaven may be found,” I might think that this applies only during the daytime, but not at night. (See Re”m for further elaboration.)

(Exodus 12:18) "On the first day, on the fourteenth day": Scripture made it mandatory (to eat matzoh on the first night). "until the twenty-first day of the month in the evening." What is the intent of this? From (Ibid. 18) "Seven days shall you eat matzoth," I would know only of the days. Whence would I derive (the same for) the nights? It is, therefore, written "until the twenty-first day," to include the nights.