Eruvin, Chapter 3, Mishnah 6. In the previous mishnah we dealt with making an eruv techumin conditionally, where a person can set it up for one of two directions and decide later which one it will be. Our mishnah discusses making two different eruvin for two different days, specifically when Yom Tov falls right next to Shabbos: may a person establish one eruv for one day and a different kind of eruv for the second day, or not.
The view of Rabbi Eliezer:
"Yom tov hasamuch l'Shabbos, bein milfanav u'vein me'acharav, me'arev adam shnei eiruvin" - whether Yom Tov falls on Friday, before Shabbos, or on Sunday, after Shabbos, a person may make two different eruvei techumin: one for the first day and one for the second day. The reason: in Rabbi Eliezer's view, the two days are two separate sanctities, and therefore they can have two different eruvin.
It should be clarified that we are not speaking about placing the eruvin at a distance that cannot be reached. The two eruv locations must be within two thousand amos of each other. For example, a person may place one eruv nine hundred amos to the east and another nine hundred amos to the west, so that he can reach both of them. For if the first eruv is located to the east of his house, he will not be able to reach the second eruv when the days change from Shabbos to Yom Tov if it is too far to the west of the house. But apart from this condition, he may have two eruvin and two different center points.
These are the wordings the mishnah lists:
"Eiruvi harishon lamizrach v'hasheini lama'arav" - the eruv of the first day is toward the east, and that of the second day toward the west.
"Harishon lama'arav v'hasheini lamizrach" - and the reverse.
"Eiruvi harishon, v'hasheini k'vnei iri" - the eruv will be in effect on the first day, and on the second day his status will be like that of the rest of the people of his city, without an eruv.
"Eiruvi hasheini, v'harishon k'vnei iri" - the eruv will be in effect on the second day, and on the first day his status will be like that of the rest of the people of his city, without an eruv.
The view of the Chachamim:
"Me'arev l'ruach achas, o eino me'arev kol ikar" - for both days together a person may make an eruv in one direction only, or he does not make an eruv at all. It is all or nothing: either one eruv for both days, or no eruv whatsoever.
The Gemara notes that this wording is repetitive, saying the same thing twice. The Gemara therefore explains that the intent of the Chachamim is: just as on a single day a person can make an eruv only in one direction, and cannot make an eruv for half the day in this direction and half the day in another direction, or else he makes no eruv at all that day, so too for two days he can make an eruv only in one direction, or make no eruv at all.
The reason for their view: the Chachamim are concerned that Shabbos and Yom Tov may be considered one sanctity, and therefore one may go in only one direction and there cannot be two separate eruvin. However, as we will see shortly, this is not a definite ruling but only a concern. They are not certain, since it may be that these are two separate sanctities, two separate days, for which two eruvin may be made, and because of the doubt they are stringent and rule that one may not make two different eruvin.
How to ensure a valid eruv for both days:
Since the Chachamim are concerned that these may be two sanctities, one may not rely on the eruv having been in its place only during the first bein hashemashos to be effective for both days; rather, the eruv must be present in its place on the second day as well.
Yom Tov that falls before Shabbos: on erev Yom Tov he brings the eruv to the eruv location and remains there with it until nightfall, and in this way the place is established as his eruv location. Afterward he takes it and returns home with it, since he does not want to leave it there and risk its being eaten, taken or ruined. On the second day, on Yom Tov before Shabbos begins, he brings it again to that same place and remains there with it until nightfall, and here he may eat it on the spot, since he cannot bring it home once Shabbos has begun. It turns out that he has gained both things: the eruv stood in its place and was effective, and he also enjoyed eating it. But he is obligated to bring it again on the second day, since perhaps these are two sanctities.
Shabbos that falls before Yom Tov: in the reverse situation he must bring the eruv to its place on erev Shabbos and leave it there, hoping that it will still be there the next day. If it was eaten, it is not valid for the second day. He returns there and checks whether the eruv is in its place, and if he finds it there, he may take it and return home with it or eat it on the spot.
"Ne'echal bayom harishon" - if the first day was Shabbos, and when he returned to the place he found that the eruv had disappeared and been eaten, his eruv is valid for the first day only, since it was in its place on erev Shabbos, and it is not an eruv for the second day.
From here Rabbi Eliezer argued against the Chachamim: you too are concerned, and you do not say that the fact that the eruv was in its place on the first day is effective for both days. If so, you concede to me that these are indeed two sanctities, since you require that it be in its place on the second day as well.
But the truth is that the Chachamim were merely concerned. They are concerned that these may be two sanctities, yet they hold that it may be one sanctity, and therefore a person cannot make two eruvin in two different directions. At the same time, even regarding the one direction he chose, he must make sure that the eruv was in its place on both days, since perhaps these really are two separate sanctities.
In summary: in this mishnah Rabbi Eliezer and the Chachamim disagree about Yom Tov that falls next to Shabbos. In Rabbi Eliezer's view the two days are two sanctities, and therefore a person may make two eruvin, one to the east and one to the west, or an eruv for one day only while on the second day his status is like the rest of the people of his city, provided that the two eruv locations are accessible from one another. The Chachamim hold "me'arev l'ruach achas o eino me'arev kol ikar," and according to the Gemara's explanation the law for two days is the same as the law for one day. Because of the doubt whether it is one sanctity or two, they are stringent in both directions: one may not make an eruv for two directions, and on the other hand one must be careful that the eruv stand in its place on both days, as was explained in the procedure for bringing the eruv in each of the two situations.