Eruvin 38A

אי אתה מודה שמא יבקע הנוד, ונמצא שותה טבלים למפרע? אמר להן: לכשיבקע.

don’t you at least concede that we must be concerned that perhaps the leather flask will burst, and retroactively this person would have been drinking tevel? R' Meir said to them: When it bursts, I will consider the matter, but now I am not concerned about this possibility.

מתני׳ רבי אליעזר אומר: יום טוב הסמוך לשבת, בין מלפניה ובין מלאחריה ⁠— מערב אדם שני עירובין, ואומר: עירובי בראשון למזרח ובשני למערב; בראשון למערב ובשני למזרח; עירובי בראשון, ובשני כבני עירי; עירובי בשני, ובראשון כבני עירי.

Mishnah: R' Eliezer says: With regard to a Festival adjacent to Shabbat, whether before it, on a Friday, or after it, on a Sunday, a person may establish two eiruvin of Shabbat borders [teḥumin] and say as follows: My eiruv on the 1st day should be to the east, and on the 2nd day to the west. Alternatively, one may say: On the 1st day it should be to the west and on the 2nd day to the east. Similarly, one may say: My eiruv should apply on the 1st day, but on the 2nd day I should be like the rest of the inhabitants of my town, or: My eiruv should apply on the 2nd day, but on the 1st day I should be like the rest of the inhabitants of my town.

וחכמים אומרים: או מערב לרוח אחת, או אינו מערב כל עיקר. או מערב לשני ימים, או אינו מערב כל עיקר.

And the Rabbis disagree and say that such a split is impossible. Rather, he either establishes an eiruv in one direction for both days, or he establishes no eiruv at all; either he establishes an eiruv for the two days, or he establishes no eiruv at all.

כיצד יעשה? מוליכו בראשון ומחשיך עליו, ונוטלו, ובא לו. בשני מחשיך עליו ואוכלו ובא לו, ונמצא משתכר בהליכתו, ומשתכר בעירובו.

What does one do to establish an eiruv that will be valid for both the Festival and Shabbat? He or his agent brings the eiruv to the location that he wishes to establish as his residence on the eve of the 1st day, and he stays there with it until nightfall, the time when the eiruv establishes that location as his residence for the Festival, and then he takes it with him and goes away, so that it will not become lost before the following evening, in which case he would not have an eiruv for the 2nd day. On the eve of the 2nd day, he takes it back to the same place as the day before, and he stays there with it until nightfall, thereby establishing his residence for Shabbat, and then he may eat the eiruv and go away, if he so desires. Consequently, he benefits in that he is permitted to walk in the direction that he desires, and he benefits in that he is permitted to eat his eiruv.