984. Making an Eiruv for Someone Else Still to Be Determined

Eiruvin 8:7

Let’s say that one person tells five others that he is making an eiruv for one of them, whom he will select later, with the result that the one he chooses will be able to travel based on the eiruv and the others may not. The one chosen may travel based on this eiruv even if the one who made it does not name the beneficiary of the eiruv until after Shabbos began. This is because retroactive selection is effective when it comes to matters of rabbinic law. Similarly, one may make an eiruv for every Shabbos of the year with the condition that he may choose to rely upon the eiruv and travel, or not rely on the eiruv and not travel like all the other residents of his city. He may then rely upon the eiruv on any Shabbos he likes even if he doesn’t make his decision until after Shabbos begins.

Eiruvin 8:8

Let’s say that someone makes an eiruv for the two days of yom tov as observed outside of Israel, or for an adjacent Shabbos and yom tov. Even if he makes one eiruv in one direction for both days, it must be accessible in its spot on both the first and the second night during the twilight period. Here's how he should proceed: He should take the eiruv to the its designated spot on the eve of the first day and wait there with it until nightfall. He may then leave, taking the eiruv with him if it is yom tov (when carrying is permitted). The next day, he should bring the eiruv to the same spot, leave it there until nightfall, then eat it if it is Shabbos or take it with him if it is yom tov. It must be there on both evenings because the two days are two separate holy days. Since they are not considered one continuous period of holiness, it is not possible to make an eiruv for both days on the first night.