Although each person has a mitzva to set aside an eruv tavshilin, there is a mitzva incumbent on the leading resident of the city – that is, the local rabbi – to set aside an eruv for everyone in the area. His eruv allows everyone to cook on Yom Tov for Shabbat, even if they themselves forgot to set aside an eruv or were unable to do so. Similarly, one who does not know how to set aside an eruv may rely upon the rabbi’s eruv. However, if one could have set aside an eruv but chose not to do so, he may not rely on the rabbi’s eruv, since he ignored the mitzva. Similarly, if one forgot to set aside an eruv twice in a row, he is considered to be negligent (poshe’a), and may not rely on the rabbi’s eruv. However, if one forgot once, then remembered, and then forgot again, he is not considered to be negligent. He may rely on the rabbi’s eruv.
In order for the eruv to benefit everybody, it must belong to everybody. To assure this, an act of acquisition (kinyan) must take place. This means the rabbi must give the eruv’s food to someone else, who lifts it one tefaḥ with the intent to acquire it on behalf of all the city’s residents (including the rabbi). Once the food belongs to all the residents, the rabbi should take the eruv and declare: “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.” It is preferable le-khatḥila for the kinyan to be effected by a respected person who is not a dependent member of the rabbi’s household. Be-di’avad, the rabbi’s wife may do it (SA 366:10; 527:10-11).
The rabbi’s eruv is effective for everyone within teḥum Shabbat, including one who was unaware that an eruv was being set aside for him. As long as he finds out on Yom Tov that the rabbi set aside an eruv for all the residents, he may rely on it to cook for Shabbat. However, someone outside of the teḥum may not rely upon the eruv, since he would not be allowed to walk over and eat it. Even if he set aside an eruv teḥumin (and could walk over), he still may not rely on the eruv tavshilin, because it is reasonable to assume that the rabbi did not have him in mind when he set it aside (SA 527:8-9).
If it is known that the local rabbi always makes sure to set aside an eruv for everybody, one who forgot may rely upon the rabbi’s eruv without checking further, as there is a ḥazaka (presumption) that he remembered to set aside the eruv. Furthermore, if the rabbi did forget, he would make a public announcement to prevent people from transgressing by cooking for Shabbat in reliance on his eruv (Rema 527:9).
In addition to the local rabbi, any resident may set aside an eruv for everyone in the area. Then, should the rabbi forget to do so, he can inform the rabbi and community that he set aside an eruv, and everyone may then rely on that eruv. The resident should make sure that someone else lifts the eruv one tefaḥ with the intent to acquire it on behalf of all the city’s residents. He should also make sure to use the formulation that explicitly includes the city’s residents (MB 527:32; SHT ad loc. 31).