It is a Torah commandment to eat matza on the night of the fifteenth of the month of Nisan, as it says, “In the evening, you shall eat matzot” (Shemot 12:18). Even though the Torah also says, “you shall eat matzot for seven days” (Shemot 12:15), the Sages inferred, based on the rules for interpreting the Torah, that the Torah does not mean to command us to eat matzot all seven days, but rather that matza is the staple food that we eat on Pesaĥ instead of bread. However, one who wants to live off of fruits, vegetables, meat, and dairy products may do so.
The simple meaning of this is that one who eats matzot all seven days of Pesaĥ does not thereby fulfill a mitzva. This is what the Sages meant when they said (Pesaĥim 120a) that eating matza during the seven days is “voluntary” (“reshut”). Yet many leading halakhic authorities have written that although eating matza is obligatory only at the Seder, and indeed that is why the Sages instituted the special blessing over the eating of matza at the Seder only, nevertheless one who eats matza on the other days of Pesaĥ still fulfills a mitzva, even if it is not obligatory. In this view, the Sages referred to eating matza on the remaining days of Pesaĥ as a “reshut” only by way of contrast with the obligation to eat matza on the night of the fifteenth. According to this view, the verse “you shall eat matzot for seven days” retains its simple meaning. This is how Ibn Ezra and Ĥizkuni explained the verse, this is implied by a statement by Rosh, and this was the practice of the Vilna Gaon. However, even they maintained that the mitzva consists of eating a kezayit (olive’s bulk) of matza at each meal, and that eating more does not add to the mitzva.
. Pesaĥim 120a explains this based on one of the rules for interpreting the Torah, namely, that any item singled out from a class of items did not leave the class to teach a new idea only about itself, but to apply the new teaching to the entire class. In this case, the “class” is “you shall eat matza for seven days.” However, the Torah says later on, “for six days you shall eat matza, and the seventh day will be a festival for the Lord, your God” (Devarim 16:8). Clearly, the “seventh day” was excluded from the general class in that there is no mitzva to eat matza on that day. This in turn teaches about the entire class – all seven days – that there is no mitzva to eat matza. Only on the night of the fifteenth, the Seder night, is there such a mitzva, based on the verse “in the evening you shall eat matzot” (Shemot 12:18).
Most Rishonim and Aĥaronim do not mention any mitzva associated with eating matza during the seven days of Pesaĥ, implying that there is in fact no such mitzva. However, many Rishonim and Aĥaronim do actually mention such a mitzva, including: Ibn Ezra on Shemot 23:15, Ĥizkuni on Shemot 12:15, and Responsa Rosh 3:23 citing the Ge’onim that one does not wear tefillin on Ĥol Ha-mo’ed Pesaĥ, since there is already a “sign” on Pesaĥ, namely, eating matza. So states MB 475:45 in the name of Gra (Ma’aseh Rav §185). See Ha-Seder He-arukh I:78:7-14.