Siman 665
Presented here for study, not as a practical ruling. The Shulchan Aruch rules by Sephardi custom, the Rema's glosses by Ashkenazi custom, and the Kitzur follows Ashkenazi custom. For a practical question, ask your rabbi.
א אֶתְרוֹג בַּשְּׁבִיעִי אָסוּר, שֶׁהֲרֵי הֻקְצָה לְכָל שִׁבְעָה, וַאֲפִלּוּ נִפְסַל אַחַר שֶׁעָשָׂה בּוֹ מִצְוָה אָסוּר כָּל שִׁבְעַת יָמִים; וּבִשְׁמִינִי עֲצֶרֶת, מֻתָּר. וּבְחוּצָה לָאָרֶץ, שֶׁעוֹשִׂים שְׁנֵי יָמִים טוֹבִים שֶׁל גָּלֻיּוֹת, אָסוּר אַף בַּשְּׁמִינִי וּמֻתָּר בַּתְּשִׁיעִי, וַאֲפִלּוּ חָל לִהְיוֹת בְּאֶחָד בְּשַׁבָּת; וְיֵשׁ אוֹסְרִים בְּחָל לִהְיוֹת בְּאֶחָד בְּשַׁבָּת.
1 It Is Forbidden to Eat the Etrog on the Seventh Day, 2 Seifim: It is forbidden to eat the etrog on the seventh day because it is dedicated [muktzeh for its mitzvah] all seven days. Even if it is invalidated after performing the mitzvah, it is forbidden all seven days. On Shmini Atzeret, it is permitted. Outside of Israel where they do two days of Yom Tov, it is also forbidden on Shmini Atzeret, though on the ninth day [Simchat Torah] it is permitted, even if it is Sunday. There is one who forbids it if it falls on a Sunday.
ב הִפְרִישׁ שִׁבְעָה אֶתְרוֹגִים לְשִׁבְעָה יָמִים, כָּל אֶחָד יוֹצֵא בּוֹ וְאוֹכְלוֹ לְמָחָר; אֲבָל בְּיוֹמוֹ אָסוּר, שֶׁהֻקְצָה לְכָל אוֹתוֹ הַיּוֹם.
2 If one dedicated seven etrogs for the seven days, one for each day, each one can be used for the obligation and then eaten the next day. On its day it is forbidden because it is muktzeh for the whole day.