When the second day of Yom Tov falls on Shabbat, or if Shabbat falls immediately after Yom Tov, it is rabbinically forbidden to cook or prepare on Yom Tov for Shabbat. When executed properly, eruv tavshilin allows one to prepare on Yom Tov for Shabbat.

Interestingly, there is no Torah prohibition to cook or bake on Yom Tov for Shabbat, even though ostensibly one may only cook on Yom Tov for Yom Tov itself.  The Gemara (Pesachim 46b) offers two reasons why this does not constitute a Torah prohibition: Shabbat and Yom Tov are considered to be one unit since Yom Tov is referred to as Shabbat in the Torah. Just as it is permitted to cook and bake on Yom Tov for Yom Tov, it is permitted to cook and bake on Yom Tov for Shabbat.

Also, when one cooks or bakes additional food on Yom Tov, it is not a Torah violation because it is possible that one will need the food for unexpected guests who might arrive on Yom Tov.

If the Torah permits the preparation of food on Yom Tov for Shabbat, why did the rabbis institute the eruv tavshilin in the first place?  The Gemara (Beitzah 15b) gives two explanations: When Yom Tov precedes Shabbat, one is prone to overlook the needs of Shabbat. The rabbis therefore created a special tangible preparation for Shabbat that must be attended to before the start of the Yom Tov so people will remember Shabbat as well. Also, If we were to permit cooking on Yom Tov for Shabbat without any reminder, one might come to cook on Yom Tov for the subsequent weekdays, which would violate a Torah prohibition.

The Procedure for Eruv Tavshilin

On erev Yom Tov one sets aside two types of food, one cooked and one baked (Mishnah Berurah 527:5–6).  If one cannot obtain both items, a cooked item alone would be acceptable but a baked item alone would not suffice.  The cooked item must be at least the size of a large olive (approximately half the size of a chicken’s egg) and the baked item should be at least the size of a chicken’s egg.

The selected items are held in one’s hands while the bracha (blessing) and subsequent Aramaic text, as they appear in the siddur, is recited.  It is necessary to understand the text as it is recited.  If one does not understand the Aramaic text, it should be recited in one’s native language (Rema 527:12).

When is the eruv tavshilin effective?  Can I eat the eruv tavshilin food?

The eruv tavshilin allows one to prepare for Shabbat only on erev Shabbat, but not on Thursday when it is the first day of Yom Tov (Shulchan Aruch 527:13).  In addition, every effort must be made to complete the preparations early enough on Friday afternoon so that the food will be edible well before Shabbat.  Nevertheless, if the preparations were left until late Friday afternoon, they may still be done (Beiur Halacha 527:1).

The food items used for the eruv tavshilin must remain intact as long as preparations are being made for Shabbat.  Perishable items used for the eruv should be stored in the refrigerator as needed.  If the eruv foods were consumed or discarded, the eruv ceases to be valid (OC 527:15).

If a challah (or matzo) is used as the baked item of the eruv, it is customary to use the challah for additional mitzvahs as one of the two loaves of lechem mishnah at each of the three Shabbat meals, and to consume the challah at the third meal of Shalosh Seudos.

If one is planning to be fully prepared for Shabbat before Yom Tov starts, is an eruv tavshilin necessary?

Rav Moshe, zt”l, understands the opinion of the Magen Avrohom (O.C. 527:1) to be that it is not absolutely necessary to make an eruv tavshilin if one is all prepared for Shabbat.  Nevertheless, Iggrot Moshe (Orach Chaim, 5:20:26) notes that even when a person is not planning to cook or bake on Yom Tov for Shabbat, an eruv tavshilin should be performed as a precaution for an unexpected need. Rav Moshe, z”tl, in Orach Chaim, 5:37:9, writes that a beracha should not be recited in such a case.

What should be done if one forgot to perform an eruv tavshilin?

In this case, it is permissible to rely on the eruv tavshilin performed by the rabbi of one’s city, as it is customary for him to have his community in mind when performing the ritual. This can only be relied upon provided eruv tavshilin was not forgotten due to negligence (Shaar Hatziyun 527:32). In addition, one cannot rely on the rabbi’s eruv for two consecutive Yamim Tovim (Kaf Hachaim 527:48). The Chayei Adam (Klal 102:7) questions whether one may ever rely on the rabbi’s eruv for a second time.

Another option is to have someone who made an eruv tavshilin cook for the one who forgot. In this case, ownership of the ingredients must be transferred to the one who is allowed to cook. This person may then proceed to cook even in the home of the person who did not make an eruv tavshilin (Shulchan Aruch, Orach Chaim 527:20).

When Yom Tov is on Thursday and Friday, one can make an eruv on Thursday if one forgot to do so before Yom Tov. Consult your local Orthodox rabbi for the text that must be recited in such instances.

Adapted from “Eruv Tavshilin Primer” by Rabbi Moshe Zywica with permission from our partner OU Kosher, the largest American kosher certification agency.