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A. Doing Eiruv Tavshilin when the second day of Rosh Hashanah falls on Erev Shabbos:
Whenever the 2nd day of Rosh Hashanah falls on Erev Shabbos one must do Eiruv Tavshilin on Erev Rosh Hashanah [Wednesday] in order to be allowed to cook on the 2nd day of Rosh Hashanah for Shabbos.
Cooking on the first day of Yom Tov for Shabbos: It is forbidden to cook foods on the first day of Yom Tov [i.e. Thursday] on behalf of Shabbos even if Eiruv Tavshilin was performed. The Eiruv Tavshilin only allows one to cook on the second day of Rosh Hashanah [i.e. Friday] on behalf of Shabbos.
Cooking with enough time so the food is ready before Shabbos: Even when Eiruv Tavshilin is performed it is only permitted to cook food for Shabbos if there is enough time for the food to be fully cooked and servable to guests on Rosh Hashanah, prior to sunset. It is Biblically forbidden to cook foods if there isn’t enough time left for the food to be served before sunset. Many are unaware of this matter.
The Eiruv Tavshilin must be performed on Erev Rosh Hashanah. It may be performed any time on Erev Rosh Hashanah. If one forgot to perform Eiruv Tavshilin on Erev Rosh Hashanah then if it is still prior to nightfall [Tzeis Hakochavim] one may still do the Eiruv Tavshilin, even if it is already past sunset. If however one remembered only after nightfall then one may no longer do the Eiruv Tavshilin on Rosh Hashanah even through making a Tnaiy.
C. How is it done?
The foods: The owner of the house takes a [whole] loaf/role of bread/Matzah the size of a Kibeiytza [which is to later be used on Shabbos for the meal] and a Kezayis of a cooked piece of meat or other food which one eats together with bread. Nevertheless initially it is best to use an honorable food, such as meat or fish.
Being Mizakeh on behalf of others: The custom is to acquire the food to all the other members of the city in order to merit them with the Mitzvah in case someone forgot to do so. This is done through placing the food into the hands of a non-family member or one’s married son, or any child above Bar/Bas Mitzvah which supports himself/herself. After the food is placed in their hands the owner of the house says “I hereby acquire [this food] to all those that want to acquire and rely on this Eiruv”. The person then lifts up the food one Tefach from its current area, hence acquiring it for the townspeople. The owner then takes back the food and recites the blessing of “Al Mitzvas Eiruv” as explained next. If there is no non-family member or married son available then one may give it to one’s wife, or one’s child who is over Bar Mitzvah, to lift up one Tefach and acquire the Eiruv to the city members. However children under the age of Bar Mitzvah which are supported by the household may not be used for this acquisition.
The blessing: One says the following blessing upon making the Eiruv Tavshilin: “…Asher Kidishanu Bemitzvosav Vetzivanu Al Mitzvas Eiruv”. After the blessing one says in a language that he understands “With this eiruv it will be permitted for us to bake and cook and insulate foods, and light candles and do all our needs for Yom Tov to Shabbos”.
D. If one forgot to perform Eiruv Tavshilin:
If one forgot to perform Eiruv Tavshilin on Erev Rosh Hashanah and it is already after nightfall [Tzeis Hakochavim] one may no longer do so even through making a Tnaiy, as explained above in B. One is to speak to a Rav in regards to how he may cook food on Erev Shabbos for the sake of Shabbos.
E. What does one do with the Eiruv Tavshilin food?
The bread and food used for the Eiruv Tavshilin must be put away in a secure area in order so it will not get eaten or destroyed prior to the completion of the Shabbos preparations on Friday. From the letter of the law, once the Shabbos preparations have been completed, one may eat the foods designated for the Eiruv Tavshilin. Nevertheless if the bread is whole, and can hence be used for Lechem Mishneh, it is a Mitzvah Min Hamuvchar to delay eating the bread until one of the Shabbos meals. Some have the custom to use the bread as Lechem Mishneh for the first and second Shabbos meal and then eat it only by the third Shabbos meal.
If the food was eaten or lost: If the cooked food was partially eaten or lost prior to completing the Shabbos preparations then it is forbidden to cook or do any more preparation on behalf of Shabbos unless a Kezayis worth of the cooked food remains. If however only the bread was eaten or lost then it remains permitted to cook and prepare on Friday for Shabbos.
Reminding the public:
In those years that Eiruv Tavshilin must be performed it is proper to place signs by the public areas [i.e. Shul; Mikveh; Website] in order to remind the public of this matter. [It is suggested to also announce this in Shul at the conclusion of Shacharis and prior to Mincha.]
Whenever the 2nd day of Rosh Hashanah falls on Erev Shabbos one performs an Eiruv Tavshilin on Erev Rosh Hashanah [Wednesday]. The owner of the house takes a whole loaf/role of bread/Matzah the size of a Kibeiytza [which is to later be used on Shabbos] and a Kezayis of a cooked piece of meat or other food which one eats together with bread. If one has another person to use to acquire the food to him on behalf of the city then the owner is to say:
“אני מזכה לכל־מי שרוצה לזכות ולסמוך על ערוב זה“
The person who is acquiring the food for the townspeople then lifts the food up one Tefach. The owner then takes back the food and recites the following blessing: [If one does not have another person to use to acquire the food to the townspeople then he is to simply hold the food and begin from here with the following blessing:]
ברוך אתה ה‘ אלוקינו מלך העולם אשר קדשנו במצותיו וצונו על מצות עירוב.
After the blessing one says in a language that he understands “With this eiruv it will be permitted for us to bake and cook and insulate foods, and light candles and to do all our needs on Yom Tov for Shabbos”.
בדין יהא שרא לנא לאפויי ולבשולי ולאטמוני ולאדלוקי שרגא ולתקנא ולמעבד כל צרכנא מיומא טבא
לשבתא לנא ולכל ישראל הדרים בעיר הזאת
 See Hiskashrus 896
 It is not possible for the 1st day of Rosh Hashanah to ever fall on Erev Shabbos. [428/1]
 527/8; M”B 527/3; See Aruch Hashulchan 527/3; Piskeiy Teshuvos 527/3 for other opinions in this matter
 Initially one may not do the Eiruv Tavshilin before Erev Rosh Hashanah. [527/2-see there for a dispute on this matter and that initially we are stringent]
 527/9; Biur Halacha 261 “Mearvin”
The reason: As the time of Bein Hashmashos is doubtful whether it is day or night, and since Eiruv Tavshilin is a Rabbinical in junction one may be lenient. [Admur ibid]
 527/22; By a regular Yom Tov that falls on Thursday, Friday one may [Bedieved] make an Eiruv Tavshilin on Thursday using a Tnaiy that “if today is Yom Tov tomorrow is a weekday etc.”. [527/21] However on R”H one cannot make an Eiruv Tavshilin being that it is all considered one long day. [527/22] See Mateh Efraim 581/56
 Siddur Admur; 527/3
 In order to be able to say Lechem Mishneh over it on Shabbos as rules Admur 527/25
 57.6 grams [Shiureiy Torah p. 177]
 Siddur Admur
 527/25; “Some are accustomed to use it for Lechem Mishneh for the first and second meal and then eat it by the third meal”; See Likkutei Sichos 16/183
 27 grams [See Shiureiy Torah p. 193]
 527/3; 527/12; It is valid whether it is cooked or roasted. [ibid] If the food is raw it is invalid.
 Admur mentions meat, fish or egg. [527/11]
 Admur in Siddur; Shlah brought in Beir Heiytiv 527/2; See Admur 527/13 regarding negating the use of leftovers
 Siddur Admur; 527/17; See also 366/13
 Siddur Admur writes this directive to the general public as does the Shlah Miseches Sukkah 246; however in 527/14 this is only mentioned regarding the Rav and leaders of the city.
 This is allowed even if the non-family member is a member of one’s household. [527/17]
 This applies even if he is supported by his parents in all matters. [527/17]
 Siddur Admur; In 527/17 Admur writes a slightly different wording
The reason: By doing so the owner officially appoints the person holding the food as a messenger to acquire the food. [Admur 527/17]
 If however he does not lift it one Tefach then he does not acquire it at all for others as less than a Tefach does not acquire. [ibid] This order [to first say the statement and then have the acquirer lift a Tefach] follows the wording in the Siddur. However in 527/17 Admur writes the opposite order, to first to lift it up a Tefach and then to say the statement.
 Admur 527/17 and 366/13 that there is a dispute amongst Poskim as to whether a wife and children above Bar and Bas Mitzvah may perform the acquisition. Admur ibid concludes that Lechatchila one should not do so although Bedieved if one did so through them it is valid.
 The reason: As every item which had one Mitzvah done with it should have an additional Mitzvah performed with it as well. [ibid]
 527/25 “Some are accustomed to use it for Lechem Mishneh for the first and second meal and then eat it by the third meal”; See Likkutei Sichos 16/183
The reason: As it is proper to do many Mitzvos with an item that was already used for a Mitzvah. [ibid]
 Aruch Hashulchan 527/14
 Alef Lamagen 581/131