(ח) וְאָכְל֥וּ אֶת־הַבָּשָׂ֖ר בַּלַּ֣יְלָה הַזֶּ֑ה צְלִי־אֵ֣שׁ וּמַצּ֔וֹת עַל־מְרֹרִ֖ים יֹאכְלֻֽהוּ׃
(8) They shall eat the flesh that same night; they shall eat it roasted over the fire, with unleavened bread and with bitter herbs.
(יח) בָּרִאשֹׁ֡ן בְּאַרְבָּעָה֩ עָשָׂ֨ר י֤וֹם לַחֹ֙דֶשׁ֙ בָּעֶ֔רֶב תֹּאכְל֖וּ מַצֹּ֑ת עַ֠ד י֣וֹם הָאֶחָ֧ד וְעֶשְׂרִ֛ים לַחֹ֖דֶשׁ בָּעָֽרֶב׃
(18) In the first month, from the fourteenth day of the month at evening, you shall eat unleavened bread until the twenty-first day of the month at evening.
(ח) שֵׁ֥שֶׁת יָמִ֖ים תֹּאכַ֣ל מַצּ֑וֹת וּבַיּ֣וֹם הַשְּׁבִיעִ֗י עֲצֶ֙רֶת֙ לַה' אֱלֹקֶ֔יךָ לֹ֥א תַעֲשֶׂ֖ה מְלָאכָֽה׃ (ס)
(8) After eating unleavened bread six days, you shall hold a solemn gathering for the LORD your God on the seventh day: you shall do no work.
מאי טעמא הוי דבר שהיה בכלל ויצא מן הכלל ללמד לא ללמד על עצמו יצא אלא ללמד על הכלל כולו יצא
What is the reason that it is optional to eat matza on the first six days of Passover as well as the seventh? The seventh day of Passover is something that was included in a generalization but was explicitly singled out to teach. According to the rules of exegesis, it was intended to teach not just about itself but about the entire generalization. In other words, the seventh day of Passover was initially included in the verse: “You shall eat matzot for seven days” (Exodus 12:15), but was excluded from this generalization by the verse: “Six days you shall eat matzot.” In accordance with the above principle, the halakha of the seventh day applies to all the other days of Passover as well. That means there is no obligation to eat matza for all seven days of the Festival, but only on the first day.
תניא כוותיה דרבא (דברים טז, ח) ששת ימים תאכל מצות וביום השביעי עצרת לה' אלקיך מה שביעי רשות אף ששת ימים רשות
The Gemara comments: It was taught in a baraita in accordance with the opinion of Rava: “Six days you shall eat matzot, and on the seventh day shall be a solemn assembly to the Lord your God” (Deuteronomy 16:8). Just as eating matza on the seventh day is merely optional, i.e., there is no obligation to eat matza on the last day of Passover, but only to avoid eating leavened bread, as the verse states: “Six days you shall eat matzot,” so too, eating matza during the first six days is optional.