Why do we eat Matzah to commemorate what we did when we left Egypt, when we were already commanded to do so long before we left?

Question

In the Haggadah, we say:

⁧מצה זו שאנוּ אוכלים, על שום מה? על שום שלא הספיק בצקם של אבותינו להחמיץ עד שנגלה עליהם מלך מלכי המלכים, הקדוש ברוך הוא, וגאלם, שנאמר: ויאפו את הבצק אשר הוציאו ממצרים עגת מצות, כי לא חמץ, כי גרשו ממצרים ולא יכלו להתמהמה, וגם צדה לא עשו להם.

THIS MATZA that we eat: what does it recall? It recalls the dough of our ancestors, which did not have time to rise before the King, King of kings, the Holy One, blessed be He, revealed Himself and redeemed them, as it is said: “They baked the dough that they had brought out of Egypt into unleavened cakes, for it had not risen, for they were cast out of Egypt and could not delay, and they made no provision for the way.”

The verse quoted is Shemot 12:39, which took place when we left Egypt, on the 15th of Nissan.

The problem is, God already told Moses and Aaron, at the beginning of the month (See Rashi on v. 3, s.v. "דברו אל כל עדת ישראל לאמר בעשר לחדש"), regarding the Pesach in Egypt (in v. 8):

⁧ואכלו את הבשר בלילה הזה צלי אש ומצות על מררים יאכלהו

They shall eat the flesh that same night; they shall eat it roasted over the fire, with unleavened bread and with bitter herbs.

Then later, regarding future Pesachs, He said (in v. 18):

⁧בראשן בארבעה עשר יום לחדש בערב תאכלו מצת עד יום האחד ועשרים לחדש בערב

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.

This verse is stating the obligation to eat Matzah on the first night of Pesach.

Why does Rabban Gamliel say that the reason we eat Matzah on Leil ha-Seder (which is the Matzah we refer to, when saying "מצה זו שאנוּ אוכלים...‏"), was because our dough didn't have time to rise when we left Egypt, when we were commanded to do so, long before that happened?

Answer 1

The answer to your question on Rabban Gamliel is that he really is quoting a verse in Deuteronomy:

לֹא־תֹאכַ֤ל עָלָיו֙ חָמֵ֔ץ שִׁבְעַ֥ת יָמִ֛ים תֹּֽאכַל־עָלָ֥יו מַצּ֖וֹת לֶ֣חֶם עֹ֑נִי כִּ֣י בְחִפָּז֗וֹן יָצָ֙אתָ֙ מֵאֶ֣רֶץ מִצְרַ֔יִם לְמַ֣עַן תִּזְכֹּר֔ אֶת־י֤וֹם צֵֽאתְךָ֙ מֵאֶ֣רֶץ מִצְרַ֔יִם כֹּ֖ל יְמֵ֥י חַיֶּֽיךָ׃
Thou shalt eat no leavened bread with it; seven days shalt thou eat unleavened bread with it, the bread of affliction; for thou camest forth out of the land of Miżrayim in haste: that thou mayst remember the day when thou camest out of the land of Miżrayim all the days of thy life.

כי בחפזון יצאת. וְלֹא הִסְפִּיק בָּצֵק לְהַחֲמִיץ וְזֶה יִהְיֶה לְךָ לְזִכָּרוֹן; וְחִפָּזוֹן לֹא שֶׁלְּךָ הָיָה אֶלָּא שֶׁל מִצְרַיִם, שֶׁכֵּן הוּא אוֹמֵר "וַתֶּחֱזַק מִצְרַיִם עַל הָעָם וְגוֹ'" (ספרי):
כי בחפזון יצאת FOR THOU CAMEST FORTH [FROM THE LAND OF EGYPT] IN HASTE, and the dough therefore had no time to become leavened, and this (the eating of unleavened bread) shall be unto you as reminder of this. The haste, spoken of here, was not on thy part, but on Egypt’s part, for so it states, (Exodus 12:33) “And Egypt was urgent upon the people [hastening to send them out of the land]” (Sifrei Devarim 130:6; cf. Berakhot 9a).

There is a difference between the ceremony the Jews conducted on the night of the Exodus, and the ceremony we were commanded to do every year afterwards in remembrance of it. That ceremony did involve eating matzot, but the fact that they did not have time to bake bread for the road was not part of the commandment. Afterwards, G-d commanded us to eat matzah for seven days to commemorate that aspect of the Exodus. Ramban Gamliel is telling us that if one did not mention this, he did not fulfill his obligation, based on the verse in Deuteronomy.

Answer 2

See Shibolei HaLeket on the Haggada, who answers that Hashem knew that the Bnei Yisrael would leave in a hurry, and therefore commanded them based on the future.

ואם תאמר הרי אנו אומרים מצה זו שאנו אוכלין על שום מה על שום שלא הספיק בציקו של אבותינו להחמיץ כך פירושו מכיון שהקב"ה רואה את העתיד צופה ומביט מה שעתיד להיות על כן צום מבערב על הנס של מחר

He also notes in the beginning of the passage that the command that they received in Egypt not to eat chametz for the week only applied to future years, as the Torah implies that they would have baked chametz if they had the time. It is that lack of time we remember (or "pre-member") in the command not to eat chametz all Pesach.