שולחן ערוך אורח חיים · סימן תרס״ה

English

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.

English

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.