May one perform Eruv Tavshilin before Erev Yom Tov or on the night before Yom Tov [i.e. Yom Tov is on Thursday night, may one perform it on Wednesday night, or prior]?
The initial obligation to perform specifically on Erev Yom Tov: Initially, the Eruv Tavshilin is to be performed specifically on Erev Yom Tov, and not on the days preceding Erev Yom Tov. If one performed it prior to Erev Yom Tov then he is to redo it on Erev Yom Tov without a blessing. Nonetheless, Bedieved, if one forgot to redo the Eruv on Erev Yom Tov, then even if it was performed several days or weeks before Erev Yom Tov, it is valid so long as it remains intact and edible. [Likewise, if one knows that he will be unable to do an Eruv Tavshilin on Erev Yom Tov, then he may do so any day beforehand, without a blessing. See Q&A regarding doing so with a Tnaiy when doing so before Erev Yom Tov.]
When on Erev Yom Tov-Night versus day: The Eruv may be performed any time on Erev Yom Tov, up until sunset, [and Bedieved, up until Tzeis Hakochavim]. Nonetheless, initially, it is to be performed after daybreak on Erev Yom Tov, although in a time of need, such as one is traveling, it may be performed starting from the night of Erev Yom Tov. [It is questionable, however, if a blessing may be recited when doing so on the night of Erev Yom Tov, although those who say a blessing have upon whom to rely.]
Initially, the Eruv is to be performed after daybreak of Erev Yom Tov, and not the night beforehand. However, in a time of need, one may do so the night beforehand, and even many days beforehand, although without a blessing. Likewise, if one suspects that one may come to forget to do so on Erev Yom Tov, he may do so beforehand with a Tnaiy, as explained in the Q&A. In all cases that the Eruv was done prior to Erev Yom Tov, then one is to try to redo it on Erev Yom Tov without a blessing [unless a Tnaiy was done, as explained in the Q&A].
What is one to do if he suspects that he may forget to do the Eruv Tavshilin on Erev Yom Tov, may he do so the night before, or several days beforehand?
One who suspects that he may come to forget to do the Eruv Tavshilin on Erev Yom Tov may do so without a blessing anytime before Erev Yom Tov, even several days or weeks beforehand, and certainly on the night before Yom Tov, on the basis of a Tnaiy [stipulation]. He may then do it again with a blessing on Erev Yom Tov if he remembers [using the same Eruv Tavshilin], or Bedieved rely on this Eruv if he forgets. The Tnaiy that he should make at the time of the early Eruv is as follows: “If I forget to do the Eruv on Erev Yom Tov, I am relying on this Eruv to fulfill my obligation. If, however I will not forget, then I am now having in mind to not fulfill my obligation with this Eruv.” It suffices to think this stipulation in one’s mind, and it is not necessary to be verbalized. If however, one did not explicitly have in mind to make this Tnaiy, to not fulfill his obligation with this early Eruv if he remembers at night, and rather he casually did the Eruv without any specific intent, then although he did not say a blessing at this time, nevertheless, he may not redo the Eruv on Erev Yom Tov with a blessing, as perhaps he has already fulfilled his obligation. He is however to redo it without a blessing, as stated above.
 Admur 527:2; Michaber 527:14; Taz 527:13 and Rosh Beitza 2:1
 Ruling according to 2nd reason of Admur ibid, which is reason of Rava in Beitza 15b, and conclusion of Admur ibid to initially suspect for this reason; Michaber 527:14; Kol Bo 59 in name of Rabbeinu Nisanel; Taz 527:13 and Rosh Beitza 2:1 that so applies according to reason of Rava
The reason: As the entire purpose of the Eruv is to remember to cook and bake delicacies for Shabbos [2nd reason ibid; Rava Beitza ibid], and if one does the Eruv before Erev Yom Tov he will not remember. [Admur ibid; Taz 527:13 and Rosh Beitza 2:1]
Other opinions: According to the 1st reason in Admur ibid, which is the opinion of Rav Ashi in Beitza ibid, it is permitted even initially to make the Eruv prior to Erev Yom Tov, as its main purpose is simply to remember the honor of Yom Tov, and this process of remembering Yom Tov can be started any day that one likes. [Admur ibid; Taz ibid; Rosh ibid]
 Admur ibid
The reason: As we do not initially rely on the first reason and hence require it to be redone, however a blessing is not repeated as we nevertheless suspect for the 1st reason that perhaps it was a valid Eruv. [See Admur ibid]
 Admur ibid; Michaber ibid; Taz ibid
The reason: As Bedieved we rely on the opinion of Rav Ashi, which is the first reason in Admur ibid. [Admur ibid; Taz ibid]
 Makor Chaim 527:8; Siach Yitzchak 242 based on Siddur Yaavetz; Tzelach Beitza 17a; Nehar Shalom 527:7; Beir Moshe 4:43; Nitei Gavriel 81:11; See the Maaneh of the Rebbe in coming footnotes which can imply that even a blessing may be recited in such a case, as it is considered for him Erev Yom Tov
 Admur 527:9; Biur Halacha 261 “Mearvin”
The reason: As the time of Bein Hashmashos is questionable whether it is day or night, and since Eruv Tavshilin is a Rabbinical in junction one may be lenient. [Admur ibid]
 See Rebbe in Toras Menachem 5747 Vol. 4 p. 426 “We find no limitation as to when it is done and some do so earlier and some do so later”; Nitei Gavriel 81:10 and 12
 Shevet Halevi 9:129; Rav SZ”A, brought in Yom Tov Kehilchaso 25 footnote 12 p. 496; Rebbe in Sichas 29th Elul 5747 “It makes sense that if it was also done on the night of Erev Rosh Hashanah it is as if he did so that day” and later reclarified in a Maaneh to the Rabbanim, printed in Koveitz Ohalei Torah Chag Hashavuos 5774 p. 129, that this is only in a time of need [In the Sicha the Rebbe stated that one may do Eiruv Tavshilin also from the night of Erev Yom Tov. The Rebbe later clarified to the Rabbanimin a handwritten response that this is only in a case that one is traveling beforehand, although under normal circumstances it is to be done on Yom Tov day, after daybreak]; See Nitei Gavriel Yom Tov Vol. 2 chapter 81 footnote 18 [writes that best to initially wait until morning]; Yom Tov Kehilchaso 25 footnote 12
The reason: It is questionable whether the night of Erev Yom Tov can be considered a time of preparation for Shabbos, and hence fulfill the second reason behind Eiruv Tavshilin or not. [See Admur 527:2 for the two reasons behind Eiruv Tavshilin and that according to the second reason, it must be performed on Erev Yom Tov itself as only then is he involved in Shabbos preparations and he will come to remember it.] Therefore, initially one should not perform the Eiruv Tavshilin at night although in a time of need, such as if one will be traveling, then it is valid even with a blessing. [Poskim ibid]
Other opinions: Some Poskim rule it may even initially be performed on the night of Erev Yom Tov. [Ruling of HaRav Yaakov Yosef z”l] On the other hand, other Poskim rule it may not be done on the night of Erev Yom Tov with a blessing. [See Siach Yitzchak 242; Yisrael Vihazmanim 22; Lehoros Nasan Moadim 3:250-4]
 See previous footnote for debate in Poskim as to whether one may even initially do so on the night of Erev Yom Tov, and that some hold that it is only valid Bedieved, and hence a blessing is not to be said, just as we rule above regarding . However, seemingly, according to the Rebbe’s conclusion, it may be done on the night of Erev Yom Tov even with a blessing, in a time of need. Vetzaruch Iyun. Perhaps one can also apply here the rule of Sfek Sfeika to allow a blessing to be said. [See Admur 489:24 that we rule Sfek Sfeika Lehachmir by Brachos, although see Siddur Admur regarding Tallis as explained in Ketzos Hashulchan 7 footnote 3] Practically, the Shevet Halevi ibid concludes that one who is not secure about this allowance should not say a blessing when doing the Eruv the night before. However, Harav Yaakov Yosef concludes that one may even initially do so with a blessing.
 So rule regarding Sefiras Haomer by Bein Hashmashos: Admur 489:12; Michaber 489:3 as explained in Taz 489:6; M”A 489:7; So rule likewise here regarding Eruv Tavshilin: Beir Moshe 4:43; Nitei Gavriel 81 footnote 22
 See Admur ibid that we suspect for both opinions and hence Safek Brachos Lihakel; This certainly applies in this case that the entire validity of the Eruv is under stipulation. See Admur 489:12; Michaber 489:3
 So rule regarding Sefiras Haomer, and the same applies here regarding the Eruv: Admur ibid; M”A ibid
The reason this Tnaiy helps even according to those who rule the Mitzvos do not need intent: As even according to those opinions who hold that Mitzvos do not need intent to fulfill ones obligation, if one specifically has in mind to not fulfill his obligation, everyone agrees he is not Yotzei, as he cannot be Yotzei against his will. Therefore, if one remembers at night of his stipulation, he is to count with a blessing. [Admur 489:12; Rama 489:3; M”A 489:8; Chok Yaakov 489:14]
Other opinions: Some Poskim rule that making a Tnaiy is useless, as one cannot questionably join the fulfillment of a Mitzvah. [Taz 489:6] It is thus questionable whether he has fulfilled his obligation even if he made a Tnaiy and remembered later on. [P”M 489 M”Z 6]