The following is the procedure for setting aside an eruv tavshilin. Taking the cooked food and the bread, one recites the following berakha: “Blessed are You, Lord our God, King of the universe, Who sanctified us with His mitzvot and commanded us concerning the mitzva of eruv” (“asher kideshanu be-mitzvotav ve-tzivanu al mitzvat eruv”). Afterward, he should recite: “With this eruv it shall be permitted to us to bake, cook, light a flame, and do everything necessary on Yom Tov for the sake of Shabbat.” This text may be recited in the original Aramaic (as found in siddurim) or in translation.
If one intends to slaughter animals on Yom Tov for Shabbat, to separate foods, or to grind spices, he should ideally mention this when setting aside the eruv. However, even if he didn’t mention it, any melakha that may be done on Yom Tov may be done for the sake of Shabbat. After all, the declaration recited when setting aside the eruv is a sweeping one: “everything necessary on Yom Tov for the sake of Shabbat.”
Even if one has no plans to cook on Yom Tov for Shabbat, he should still set aside an eruv and recite the berakha, because the primary goal of the eruv is to allow for the possibility of cooking, so it is relevant even if he does not end up doing so. Additionally, by setting aside the eruv he remains aware that Shabbat is coming, and will make certain to prepare food to enjoy its three meals. Besides, according to most poskim, it is having an eruv which allows one to light Shabbat candles on Yom Tov.
When a head of household sets aside an eruv, all family members and any guests sleeping there are thereby allowed to cook and bake for Shabbat. It is even permissible for a head of household to appoint a family member or guest to set aside the eruv on everyone’s behalf. Likewise, guests in a kosher hotel, who are eating the food from the hotel’s kitchen, are all covered by the hotel’s eruv and are permitted to light Shabbat candles on Yom Tov. This is also true in a yeshiva, where all the students and their guests may rely on the yeshiva’s eruv.
The eruv is set aside before Yom Tov and is preferably made of food cooked on Erev Yom Tov for Shabbat. This way, the eruv serves as a reminder that it is prohibited to cook on Yom Tov for the upcoming week, and that nice food should be saved for Shabbat. If one sets aside the eruv any earlier, it is less likely to work as a reminder. However, be-di’avad even if one sets aside the eruv long before Yom Tov, it is effective, because his intention is for the food to serve as an eruv for the holiday. Even if one intends to use an eruv for a number of holidays, it is effective be-di’avad as long as it remains in existence (SA 527:14).
Some maintain that an eruv tavshilin permits only preparations for the Shabbat meals, while other Shabbat preparations are forbidden. This is why SA states (528:2) that even if one set aside an eruv tavshilin before Yom Tov, on Yom Tov he may not make an eruv ḥatzerot or eruv teḥumin for Shabbat (MA ad loc. 2; Yam Shel Shlomo). However, others explain SA differently: an eruv tavshilin allows one to do whatever one may do on Yom Tov for Yom Tov on Yom Tov for Shabbat. Since one may not set aside an eruv ḥatzerot or teḥumin on Yom Tov for Yom Tov, it is forbidden to do so for Shabbat as well (R. Akiva Eger; this is also the inclination of Yeshu’ot Yaakov 528:1). In their opinion, it is permissible for one who set aside an eruv tavshilin to do all of the following on Yom Tov: heat up water in order to immerse in a mikveh on Shabbat, fold a talit, and roll a Torah scroll from the Yom Tov reading to the Shabbat reading. Since this is a disagreement about something rabbinic (after taking ho’il into account), we can be lenient. This is the opinion of Ḥeshev Ha-efod 2:65; Or Le-Tziyon 3:22:6 (at the end); and Ḥazon Ovadia, p. 320.
. According to Behag, Or Zaru’a, Rosh, Rashba, and Ran, one may not light Shabbat candles without an eruv tavshilin, while Beit Yosef (basing himself on Rif and Rambam) maintains that it is permitted to light Shabbat candles even without an eruv. It is proper to defer to the stringent opinion (MB 527:55; Kaf Ha-ḥayim ad loc. 112). A single candle may still be lit, as we wrote in section 5 above.
According to Ma’amar Mordechai 527:18 and Kaf Ha-ḥayim ad loc. 113, one who does not plan to cook on Yom Tov for Shabbat should not recite the berakha when setting aside the eruv, as some say an eruv is not necessary to permit the lighting. However, the custom in practice is to recite the berakha, because the purpose of an eruv is to allow for the possibility of cooking (Ḥut Shani, p. 150). Additionally, some maintain that we take into account Rava’s opinion that the reason we set aside an eruv is to remember Shabbat (Taz ad loc. 13; this is also implied by a number of Rishonim).
. The head of household’s eruv covers all members of the household (Yam Shel Shlomo; MB 527:56). This includes married children who are visiting (Eshel Avraham [Buczacz] §7; Ḥazon Ovadia, p. 277 n. 8), as well as any other guests (Ma’amar Mordechai, Mo’adim, p. 127). This is also the case for hotel guests and yeshiva students (Ḥut Shani, p. 155). In all these cases, some disagree, as is discussed in Harḥavot 8:3:5-7. However, their reasoning is not convincing. Besides, this is a case of uncertainty about rabbinic law, so we follow the lenient view.