When Yom Tov falls on Friday, by Biblical law, one is permitted to do melachah on Yom Tov for Shabbos. In other words, one would be able to cook food on Yom Tov that will be needed for Shabbos. The Gemara (Pesachim 46b) presents two opinions as to why this should be permitted. One opinion says it is based on the principle of “ho’il” (literally, “since”) – since the food prepared on Yom Tov could theoretically be enjoyed on Yom Tov, it can be considered to be a Yom Tov preparation. Therefore, in most cases, the prohibition of doing melachah on Yom Tov for the subsequent day is only rabbinic.
The other opinion says that performing melachah on Yom Tov for the sake of Shabbos is inherently permitted at a Biblical level, unlike performing melachah on Yom Tov for a weekday.
Now, even according to the first reason (which relies on ho’il), it would seem that if Shabbos falls immediately after Yom Tov, or when the second day of Yom Tov falls on Shabbos, it should be completely permitted to cook or bake on Yom Tov for Shabbos. Any rabbinic prohibition there may be against doing melachah should really be waived because of the need to prepare for Shabbos.
Nevertheless, cooking or baking on Yom Tov for Shabbos is still prohibited, by a rabbinic ordinance, unless an eiruv tavshilin (which consists of designating prepared food, as will be defined below) was set on erev Yom Tov. The Gemara (Beitzah 15b) offers two explanations for this ordinance:129
To guard the honor of Shabbos. Chazal were concerned that when Yom Tov falls on Friday, people will focus on enjoying Yom Tov and will not leave over food that is fitting for Shabbos. Thus, Chazal required one to make an eiruv tavshilin before Yom Tov as a reminder to make sure there are appropriate dishes for Shabbos as well.
To guard the sanctity of Yom Tov. Chazal feared that were it permitted to cook on Yom Tov for Shabbos without it being manifestly obvious that the cooking is being done for Shabbos, some people might assume that it is permitted to cook on Yom Tov even for the weekday, which is strictly prohibited. The purpose of the eiruv, then, is to serve as a reminder that on Yom Tov it is permitted to cook only for Shabbos and not for a weekday.
The Process of Setting an Eiruv Tavshilin
Two articles of food, one cooked and one baked,130 are designated and held in one’s right hand.131
One should stand132 and recite the berachah:
בָּרוּך אַתָּה ה' אֱלֹקֵינוּ מֶלֶך הָעוֹלָם אֲשֶׁר קִדְּשָׁנוּ בְּמִצְוֹתָיו וְצִוָנוּ עַל מִצְוַת עֵירוּב.
After reciting the berachah, the following text is immediately recited:
בַּהַדֵין עֵירוּבָא יְהֵא שָׁרֵא לָן לַאֲפוּיֵי וּלְבַשּׁוּלֵי וּלְאַטְמוּנֵי וּלְאַדְלוּקֵי שְׁרָגָא וּלְאַפוּקֵי וּלְמִשְׁחַט133 וּלְתַקָּנָא וּלְמֶעְבַּד כָּל צָרְכָנָא מִיּוֹמָא טָבָא לְשַׁבְּתָא.
This is translated as:
“With this eiruv, it will be permitted for us to bake, cook, insulate, light lamps, transport, perform shechitah, prepare, and do anything necessary on Yom Tov for Shabbos.”
By following this procedure, one establishes the designated foods as his eiruv tavshilin.
Further Details About Making the Eiruv Tavshilin
Since the meaning of the text must be understood in order for the eiruv to be valid, the text should be recited in a language that one understands.134 Therefore, if an English speaker does not understand the Aramaic, he should recite the English translation instead.
The eiruv should be held the entire time that one is reciting the berachah and reciting the text.135
The cooked food should be at least the size of a kezayis. A standard, medium-size hardboiled egg is definitely large enough.136 The baked food should be at least a kebeitzah.137
Meat, fish, or eggs are all valid as the cooked food.138 In fact, many different types of foods are valid. However, a starch – such as grains or potatoes – may not be used.139 It is suggested that a person use either meat, fish, or eggs unless he is certain that the particular type of food he is using is valid.
If the eiruv should spoil, it is considered as if no eiruv was made.140 It is therefore suggested that the cooked food be refrigerated.
Optimally, the eiruv is prepared and the berachah and text are recited on erev Yom Tov, during the day, and not before.141 Some Poskim permit the eiruv to be set on the night of erev Yom Tov (i.e., the night preceding Yom Tov night),142 while others allow this only under extenuating circumstances.143 Additionally, it should be made before the Yom Tov candles are lit, especially if Shehecheyanu will be recited upon lighting.144
What Would Be Considered an Eiruv Tavshilin Bedieved, and What Would Not
Bedieved, even if one set the eiruv using only a cooked food, the eiruv is valid even to allow for baking. However, if one prepared only a baked item, he may not rely on this, even for baking.145
If the berachah was omitted but the text recited, the eiruv is valid.146
If the text was omitted, it is questionable whether the eiruv is valid. If one remembered before Yom Tov that he did not recite the text, he should repeat the process of making the eiruv, but without repeating the berachah.147 If he remembered only after the onset of Yom Tov, a Rav should be consulted.
129 Beitzah 16b. As far as practical halachah, the second reason is the dominant one (see Sha’ar Hatziyun 527, note 66).
130 The cooked item allows one to cook on Yom Tov, and the baked item allows one to bake on Yom Tov. If a person is planning to cook for Shabbos, but not to bake, he needs to prepare only a cooked food, but not a baked food, for eiruv tavshilin (Mishnah Berurah 527, note 6).
131 See Mishnah Berurah 206, note 18.
132 Mishnah Berurah 8, note 2.
133 Based on Mishnah Berurah 527, notes 36-37. Some Siddurim omit the words ולאפוקי ולמשחט (to transport, and to perform shechitah).
134 Mishnah Berurah, note 40, ad loc.
135 Teshuvos Maharsham II:36.
136 See Sefer Kezayis (Bodner) that a bit more than half of an extra-large egg is a kezayis according to all opinions.
137 Shulchan Aruch and Rema, Chapter 527, Passage 3. The discrepancy between the required size for the cooked and baked food is based on a Yerushalmi that is not found in contemporary editions of the Yerushalmi, but cited by some Rishonim. See Binah Le’itim (Eibshitz) Yom Tov 6:3 for an explanation of the required size of kebeitzah specifically for the bread.
137 See Teshuvos Vehanhagos VI:122:3 who notes that, lechatchilah, one machine matzah may not suffice for the eiruv tavshilin, since it might not be a kebeitzah. However, two matzos definitely suffice.
138 Mishnah Berurah, note 11, ad loc.
139 Sha’ar Hatziyun 527, note 21.
140 Aruch Hashulchan 527:13.
141 Mishnah Berurah, note 44, ad loc.
142 R’ Yosef Shalom Elyashiv, cited in Koveitz Mevakshei Torah, Yom Tov, vol. I, page 216.
143 Shevet HaLevi IX, 129:4 (cited in Eiruv Tavshilin He’aruch II:16:3). However, he seems to rule that the basic halachah would permit reciting a berachah in this instance.
144 See Shalmei Todah, Eiruv Tavshilin 7:2.
145 Mishnah Berurah 527, note 7.
146 Mishnah Berurah ibid., note 64.
147 Mishnah Berurah ibid., 63.