1) The basis for the Tosfos is the Gemara later (46b) where Rebbi Yochanan said that the Esrog is forbidden on Day 7 and permitted on Day 8, while the Sukah is forbidden even on Day 8. Further down on 46b, Rav Papa asked Abaye what is the difference between Esrog and Sukah. Abaye answered that the difference is that if one happens to want to eat a meal right at the end of the 7th day -- in the Bein ha'Shemashos twilight period -- then he must still eat the meal in the Sukah because there is a doubt and it is possible that Bein ha'Shemashos may be considered still as Sukos. We therefore say that the Sukah is Muktzah for the entire Bein ha'Shemashos period, and since it is Muktzah for Bein ha'Shemashos ("Migo d'Etzkatzai") it automatically becomes Muktzah for the entire Day 8. In contrast the Esrog is not useful during Bein ha'Shemashos, because one performed the Mitzvah with the Esrog earlier in the day, and therefore the Esrog does not become forbidden on Day 8.

2) Tosfos then states his well-known rule, as you write Reb Aryeh Leib, that "Migo d'Etzkatzai" does not forbid the following day due to the previous day. I think what might be confusing you is the continuation of Tosfos at this point, because Tosfos now proceeds to cite sources for his rule but does not yet ask how the rule is reconciled with the Gemara (46b) about the Sukah. It is only much later that Tosfos returns to the question of the Sukah. This is on line 36 of the Tosfos (counting from the top of the page). Tosfos writes "v'Kol Hani Lo Dami l'Sukah" -- "all of these cases [where we do not say Muktzah because of the previous day] are not similar to the Halachah about Sukah." It is only afterwards (after the "Alef" of the Bach) that Tosfos actually explains what the difference is.

3) Tosfos writes that Sukah is different because one is obligated to eat in the Sukah "Machmas Mah she'Yiheyeh." The wording here is slightly confusing. It means that the Mitzvah of Sukah still applies during Bein ha'Shemashos at the end of Day 7; for whatever reason, one still has to eat there. In contrast, all of the other cases he has cited above do not actually involve a Mitzvah that one is obligated to do at the end of the day.

4) In short, the Mitzvah of Sukah still applies at the end of Day 7 while the Mitzvah of Esrog has already finished at that stage, since one did the Mitzvah earlier in the day. The rule that "Migo d'Etzkatzai" does not forbid the following day due to the previous day does not apply to Sukah, because the Mitzvah of Sukah still applies right up to the end of Day 7, even during Bein ha'Shemashos, while in all the cases that Tosfos cites there is no Mitzvah present at the end of the day.

Kol Tuv,

Dovid Bloom

1) After writing the above I was still somewhat concerned about the phrase in Tosfos, "Machmas Mah she'Yiheyeh," which is rather unclear, so I did a computer search for these words. I found the Sefer "Nstan Piryo" (by Rav Nasan Gestetner zt'l of Bnei Brak, who was Niftar a few years ago). On 10b of Maseches Sukah he discusses our Tosfos and writes that the phrase "Machmas Mah she'Yiheyeh" is "Megumgemes" -- the words are unclear.

2) The Nasan Piryo cites the Rosh later in the fourth Perek (46b, Siman 5), who cites our Tosfos, but there is a slight difference in the words that he cites. The Rosh does not write that if one wants to eat during Bein ha'Shemashos of Day 7 that he must eat in the Sukah "Machmas Mah she'Yiheyeh," as Tosfos writes, but rather his wording is "me'Eizeh Ta'ama she'Yiheyeh" -- "for whatever reason it might be." It seems that the Rosh is learning as follows, and this must be the way he understands Tosfos.

3) Tosfos writes that at Bein ha'Shemashos of day 7, even though one must eat in the Sukah only because of the doubt that it might still be day 7, nevertheless the fact is that he is still obligated to eat in the Sukah. The Rosh explains this to mean that since, for whatever reason, he still needs to eat in the Sukah, the fact is that he still requires the Sukah. In other words, even though we are not certain that it is still Sukos, in practical terms the Sukah is still functional and he must eat there. It does not matter for what reason he must eat there; the reality is that he still needs the Sukah, so even though, in theoretical terms, Bein ha'Shemashos of day 7 is no longer a full-fledged day of Sukos, nevertheless in practical terms when it comes to eating in the Sukah the Sukah is just as essential as it was in the middle of Yom Tov.

4) The Rosh then adds the following phrase, which does not appear in Tosfos at all: "He must do a postive action during Bein ha'Shemashos by eating in the Sukah." The Korban Nesanel #2 explains based on the Gemara in Shabbos 45a. The Gemara there says that even Rebbi Shimon, who in general does not hold of the prohibition of Muktzah, agrees that if a person was "Dachyei b'Yadayim" -- if he did a positive action to put an item out of his domain -- the item indeed becomes subject to the prohibition of Muktzah. Similarly, the person who had to eat in the Sukah during Bein ha'Shemashos is considered as having done a positive action to show that the Sukah is still a Sukah and is still holy. Therefore, the Sukah is Muktzah on Day 8 also, because he did something active at Bein ha'Shemashos before Day 8 to show that it is still holy.

5) All of this does not apply to Esrog. Once he has taken the Esrog on the morning of the 7th day, he has no further need for it. Therefore, when Bein ha'Shemashos at the end of day 7 arrives, the Esrog has no holiness any more because it is no longer useful for Mitzvos.

(See further in Nasan Piryo how he explains the difference between Sukah and Esrog according to Tosfos.)

Kol Tuv,

Dovid Bloom