Talmud - Eruvin 41a עירובין מ״א א׳

and so too, on the eve of the Ninth of Av that occurs on Shabbat, one need not reduce the amount of food he eats; rather, he may eat and drink as much as he requires and bring to his table a meal even like that of King Solomon in his time. If the Ninth of Av occurs on Shabbat eve, we bring him an egg-bulk of food toward end of the day, and he eats it, so that he not enter Shabbat in a state of affliction.

It was taught in a baraita that Rabbi Yehuda said: We were once sitting before Rabbi Akiva, and it was the Ninth of Av that occurs on Shabbat eve, and they brought him a slightly cooked egg, and he swallowed it without salt. And it was not that he desired it so much that he ate it; rather, he did so to show the students the halakha that one need not complete the fast when the Ninth of Av occurs on Shabbat eve, so as not enter Shabbat in a state of affliction.

And Rabbi Yosei says: He must fast and complete the fast. Rabbi Yosei said to the other Sages: Don’t you agree with me with regard to the Ninth of Av that occurs on Sunday, that one must stop eating on Shabbat while it is still day? They said to him: Indeed, we agree. Rabbi Yosei said to them: What is the difference to me between entering Shabbat in a state of affliction and leaving it in a state of affliction? If one stops eating before Shabbat is over, he is spending part of Shabbat fasting, and yet even the Sages concede that one must do so.

They said to him: There is a difference. If you said that one may leave Shabbat in a state of affliction, that is because he ate and drank the entire day and will not suffer if he fasts a few minutes at the end of the day. Can you say that it is the same to enter Shabbat in a state of affliction, when he has not eaten or drunk anything the entire day?

And Ulla said: The halakha is in accordance with the opinion of Rabbi Yosei, and on the Ninth of Av that occurs on Shabbat eve one must complete the fast. The Gemara poses a question: Do we really act in accordance with the opinion of Rabbi Yosei? And the Gemara raises a contradiction based upon the following mishna: We do not initially decree a fast upon the public on the New Moon, Hanukkah, or Purim, and if the community had already begun a cycle of fasts and one of them fell out on one of these days, they do not interrupt the series; this is the statement of Rabban Gamliel. Rabbi Meir said: Even though Rabban Gamliel said that they do not interrupt the series, he conceded that they do not complete the fast on one of these days, and so too, the fast on the Ninth of Av that occurs on Shabbat eve is not completed.

And it was taught in a related baraita: Following the death of Rabban Gamliel, Rabbi Yehoshua entered the study hall to annul Rabban Gamliel’s statement with regard to fasts. Rabbi Yoḥanan ben Nuri stood on his feet and said: I see that the appropriate policy is that the body must follow the head, i.e., we must follow the statements of the earlier authorities and not deviate from established halakha. All of Rabban Gamliel’s life we established the halakha in accordance with his opinion, and now you seek to annul his statement? Yehoshua, we do not listen to you, as the halakha has already been established in accordance with the opinion of Rabban Gamliel. And there was no one who disputed this statement in any way. Therefore, this baraita demonstrates that when the Ninth of Av occurs on Shabbat eve, one must observe the fast but not complete it, and this was the accepted practice.

The Gemara resolves the difficulty, arguing that this proof is not conclusive: Indeed, in the generation of Rabban Gamliel they acted in accordance with the opinion of Rabban Gamliel, but in the generation of Rabbi Yosei they acted in accordance with the opinion of Rabbi Yosei, and from then on, the halakha follows his view.

The Gemara asks: And is it correct that in the generation of Rabban Gamliel they acted in accordance with the opinion of Rabban Gamliel? Wasn’t it taught in a baraita that Rabbi Elazar ben Tzadok, a contemporary of Rabban Gamliel, said: I am a descendant of Sena’av ben Binyamin, who observed a family festival on the tenth of Av. One time, the Ninth of Av occurred on Shabbat, and we postponed it until after Shabbat, as we do not observe the fast on Shabbat, and we fasted on Sunday but did not complete the fast because that day was our Festival. This indicates that the reason they did not complete the fast is that the day itself was a Festival for them, but on the eve of a Festival, they would indeed complete it. This proves that even in the generation of Rabban Gamliel, they did complete fasts on the eve of Shabbat and Festivals.

Ravina said that this story poses no difficulty: A rabbinic Festival is different, as they are not as stringent as Shabbat or Festivals stated in the Torah, and the festival of the family of Sena’av was not a Festival from the Torah, but one established by the Sages. Since one may fast on such a Festival for a number of hours, i.e., one may fast on it for part of the day, one also completes a fast observed on the eve of such a Festival until the evening. With regard to Shabbat, however, since one may not fast on it even for several hours, one does not complete a fast observed on Shabbat eve.

Rav Yosef said: I did not hear this ruling that the halakha is in accordance with the opinion of Rabbi Yosei. Rav Yosef had fallen ill and forgotten his learning and so was unable to remember that such a ruling had been issued. His student, Abaye, said to him: You yourself told us this halakha, and it was with regard to this point that you told it to us, as we learned in a mishna: We do not initially decree a fast upon the public on the New Moon, on Hanukkah, or on Purim. Rabbi Meir said: Even though Rabban Gamliel said that if the community had already begun a cycle of fasts, they do not interrupt the series, he conceded that they do not complete the fast on one of these days, and similarly, the fast of the Ninth of Av that occurs on Shabbat eve is not completed. And we said with regard to this mishna that Rav Yehuda said that Rav said: This is the statement that Rabbi Meir said in the name of Rabban Gamliel. But the Rabbis say: One must fast and complete the fast.

What? Does the Rabbis’ ruling that one must complete the fast not refer to all the cases mentioned in the mishna, including that of the Ninth of Av that occurs on Shabbat eve? No, it was stated only with regard to Hanukkah and Purim, but one would not complete a fast on Shabbat eve.

The Gemara comments: So too, it is reasonable to explain that this ruling does not apply to Shabbat eve,

Traduction : The William Davidson digital edition of the Koren Noé Talmud, with commentary by Rabbi Adin Even-Israel Steinsaltz.

Eruvin 41a explores whether one must complete the fast when the Ninth of Av falls on the eve of Shabbat, featuring a debate between Rabbi Yehuda and Rabbi Yosei.