What is eiruv tavshilin? We are permitted to cook and prepare foods on Yom Tov but only for that day of Yom Tov. We may not prepare foods for another day. This means that we are not permitted to prepare food on Friday for Shabbos, when Friday is a Yom Tov. However, if we begin preparing for Shabbos before Yom Tov begins, we may finish those preparations on Yom Tov. The eiruv tavshilin foods are the start of this preparations and should include both a cooked food and a baked food. Without a cooked food, the eiruv is invalid.
The eiruv permits preparation of food for Shabbos on Friday but not on Thursday.
Which activities does the eiruv permit? Only those activities that are permitted on Yom Tov, e.g. cooking, grinding, sorting. It does not permit switching on lights.
Who has to make an eiruv tavshilin? The head of the household makes an eiruv which includes the entire family.
Who need not prepare an eiruv tavshilin? If we will not be preparing foods on Friday for Shabbos and need not kindle Shabbos lights, an eiruv is not required. If we need only kindle Shabbos lights, an eiruv is required but the b’racha is not recited.
Is a visitor included in the host’s eiruv? A guest who relies entirely upon the host for their Yom Tov and Shabbos requirements, need not prepare an eiruv. A visitor who can participate in the family eiruv by acquiring a share in the food.
Which b’racha is normally recited? ברוך אתה ה‘ א-להינו מלך העולם אשר קדשנו במצותיו וצונו על מצות עירוב. Following this a declaration is made which describes the purpose of the eiruv. It is not a meaningless ritual and should therefore be said in a language that one understands: By means of these eiruv foods, we will be permitted to bake, cook, keep foods warm, light candles, carry, and do all that we need on Yom Tov for Shabbos. Traditionally it is said in Aramaic: בהדין עירובא יהא שרא לנא לאפויי ולבשולי ולאטמוני ולאדלוקי שרגא ולאפוקי ולמעבד כל צרכנא מיומא טבא לשבתא. There is a longer a longer version which includes those who have not made an eiruv.
The eiruv foods ought to be held when reciting the b’racha and declaration.
The Eiruv should be prepared the afternoon before Yom Tov but may even be made on the preceding night. It may be made even after candle lighting provided it is still before nightfall.
If one does not have any fresh food, one may use frozen food that is fully cooked.
The eiruv foods must be kept until Shabbos.
The town Rav includes in his eiruv all those who did not make their own eiruv. In an emergency, one may give his food to another who has made an eiruv who may make all the preparations required for Shabbos. Shabbos candles may be lit in the same manner.