Siman 393

One may not establish eruvei chatzerot or shitufei mevo'ot on a Yom Tov that falls on the eve of Shabbat. And even though on Yom Tov itself there is no eruvei chatzerot, and regarding this matter it is like a weekday—nevertheless, it is forbidden to establish an eruv on Yom Tov, as it is like one who fixes something on Yom Tov (Rashi, Beitzah 16b). There is also in this a matter of hachana, for he prepares from Yom Tov for Shabbat the permission to carry, and Yom Tov does not prepare for Shabbat (Tosafot, Eruvin 38a, s.v. "V'Rabbi Eliezer"), but rather he should establish the eruv on the eve of Yom Tov. And if he did not establish the eruv on the eve of Yom Tov and remembered on the first day of Yom Tov, if Yom Tov fell on Thursday and Friday—he should establish the eruv on the first day of Yom Tov with a condition, by taking the loaf in his hand and saying: If today is a weekday—this is the eruv, and if today is holy—my words are nothing. And on the next day he should take this loaf and say: If today is holy—I have already established the eruv yesterday, and if today is a weekday—let this be the eruv. And these words apply to the two days of Yom Tov of the Diaspora, such as Pesach, Shavuot, and Sukkot. But on the Yom Tov of Rosh Hashanah—it is forbidden even with a condition, for it is considered like a long day, and both of the two days are one sanctity. But regarding the Yom Tov of the Diaspora, only one day is holy, and out of doubt we observe two days, as is known.

In the Tur it is written that on the next day he should take that same bread that he took yesterday. And certainly this is not precise, for what difference does it make to me if it is this bread or another bread, (Beit Yosef) rather it is good advice, for he is forbidden to eat it until Shabbat begins, so why should he be careful with two loaves. (Magen Avraham) Furthermore, since the mitzvah was performed with it yesterday—let it be performed with it today as well. (Turei Zahav) And it is simple that "with this eruv"—he must say on both days, for on the first day he said: "If today is holy—my words are nothing," so how can he rely on this tomorrow. And a blessing—he should not bless neither on the first day nor on the second day, for how can he bless on the first day, since he says: "If today is holy—my words are nothing," upon what would he bless, and likewise on the next day. And if we say that he should bless on condition: "Who has sanctified us with His commandments and commanded us to make an eruv on condition"—we have not found this blessing in any place, and so it is below in siman 527 regarding eruvei tavshilin. And there is one who wrote there that he should bless, (Magen Avraham ibid. subsection 25) and these matters are wondrous in my eyes.

And know, that the Rambam in chapter 6 of Yom Tov, law 15, wrote: That nowadays, since we are experts in the fixing of the month, and the first day of Yom Tov is a certain Yom Tov, and this that we observe a second day of Yom Tov is merely a custom, for they sent from there "be careful with the custom of your fathers," as is explained in the Gemara (Beitzah 4b) — one may not set an eruv with a condition, for how can he say "if today is a weekday etc.," when we know that today is holy. And the Ra'avad, the Rosh, and the Ran disagreed with him, for even in the time of the Gemara they were experts in the fixing of the month, and nevertheless they commanded to place it with a condition, and also the Rif brought this as the halacha. And on the contrary, according to his words, why should we not place the eruv on the second day of Yom Tov, since we know it is a weekday. Rather, perforce, since we were commanded to conduct it as a Yom Tov — it is forbidden to show that it is a weekday, and if so, the original enactment automatically remains in its place, and in the Shulchan Aruch they did not bring his words at all.

Both eruvei chatzerot and shitufei mevo'ot—one may establish them during bein hashemashot, for since it is a rabbinic matter, a doubt is ruled leniently. And even if he has already accepted upon himself the addition to Shabbat, nevertheless, in essence it is weekday, and he may establish the eruv. And some prohibit it when he has accepted upon himself the addition to Shabbat, since for him the sanctity of Shabbat has taken effect, how can he treat it as weekday? And even if another person establishes the eruv, it appears that for him it is not effective according to this opinion, and from Tosefta Sukkah, last chapter, there is a proof for this opinion. (The Gra; and the difficulty raised by the Taz has no understanding, see there and examine carefully.)

If one established an eruv for two people—for one he established the eruv while it was yet day, and his eruv was consumed during bein hashemashot, and for the other he established the eruv during bein hashemashot, and his eruv was consumed after it grew dark—the sages of blessed memory said (Shabbat 34a) that both of them have acquired the eruv. For regarding the one whose eruv was consumed during bein hashemashot, we treat it as night, and for the one who placed it during bein hashemashot, we treat it as day. And this is not a case of "two contradictory rulings," since this pertains to two different people, and a doubt regarding a rabbinic law is ruled leniently. And even though it was performed by a single agent, nevertheless, the matter does not affect the agent but rather the principals. However, if he established the eruv during bein hashemashot and it was consumed during bein hashemashot, it is not an eruv. And even though one could say that at the time he placed it, it was day, and at the time it was consumed, it was night—nevertheless, all of bein hashemashot is a single doubt, and regarding one person it is impossible to say "two contradictory rulings," and the sages did not grant such a great leniency as to treat a single man as partly in the day and partly in the night. (Beit Yosef in the name of the Ri)