Deuteronomy 16:8

ששת ימים תאכל מצות SIX DAYS THOU SHALT EAT UNLEAVENED BREAD — But in another passage it states, (Exodus 12:15): “seven days [ye shall eat unleavened bread]’’! But the explanation is: seven days ye may eat Mazzoth prepared from the old produce and six days (out of the seven. i.e. the last six days, after the Omer has been offered and the new crop has become permitted as food) ye may eat Mazzoth prepared from the new crop (Sifrei Devarim 134:5). Another explanation is: It teaches regarding the eating of unleavened bread on the seventh day of Passover that it is not obligatory; and from here (from this law concerning the seventh day) you may derive the law for the other six days. For the seventh day was included in the general statement (“seven days, thou shalt eat unleavened bread’’), and in the text: “six days thou shall eat unleavened bread” it has left the general statement, to teach ... eating unleavened bread on it is not obligatory but optional. Now, according to the well-known rule, it did not leave the general statement in order to teach this regarding itself alone but regarding everything that is included in the general statement. Now how is it with the seventh day? It is optional as regards the eating of unleavened bread (as explained in the earlier portion of this comment)! This, too, according to the rule, applies also to everything that was included in the general statement, and therefore all the other days are also optional in this respect, with the exception, however, of the first night of Passover, for which Scripture has fixed it (the eating of unleavened bread) as an obligation, as it is said, (Exodus 22:18) “at evening ye shall eat unleavened bread” (Pesachim 120a; cf. also Rashi on Exodus 12:15). עצרת לה' אלהיך [AND ON THE SEVENTH DAY SHALL BE] A RESTRICTION IN HONOR OF THE LORD — i.e., restrict yourself from work (Chagigh 18a). — Another explanation is that עצרת denotes a gathering for eating and drinking (a banquet), like the expression used (Judges 13:15) “Let us detain (נעצרה) thee [that we may make ready a kid for thee]”.

שֵׁ֥שֶׁת יָמִ֖ים תֹּאכַ֣ל מַצּ֑וֹת וּבַיּ֣וֹם הַשְּׁבִיעִ֗י עֲצֶ֙רֶת֙ לַיהֹוָ֣ה אֱלֹהֶ֔יךָ לֹ֥א תַעֲשֶׂ֖ה מְלָאכָֽה׃ {ס}        

Six days thou shalt eat unleavened bread, and on the seventh day shall be a solemn assembly to the Lord thy God: thou shalt do no work.