Explanation (R. Chananel): We have a tradition that R. Akiva was dangerously sick that year, and the doctors brought to him to eat a roasted egg towards the end of the day;

R. Yehudah was not meticulous [to know the reason]. Therefore, he relied on what he saw, and did not know the primary reason why they did so;

Tamuz of that year was full, therefore Tish'ah b'Av fell on Erev Shabbos. They made Av short, and Rosh Hashanah fell on Shabbos. This is from Tosfos Shantz. (R. Chananel added that Pesach was on Thursday that year. It seems that he holds like the Ri (in Tosfos Pesachim 58b DH k'Ilu), that even when they were Mekadesh the month based on witnesses, they ensured that Pesach not fall on Monday, Wednesday or Friday, and Rosh Hashanah not fall on Sunday, Wednesday or Friday.)

Question: Shmuel said in Pesachim (54b) that Bein ha'Shemashos of Tish'ah b'Av is permitted! (If so, one need not stop eating while it is still day!)

Answer #1: In any case, [the fast] starts while it is still day, for one cannot know exactly the beginning of the night. (Perhaps Tosfos means that one starts while it is Safek day, for one cannot know exactly the end of Bein ha'Shemashos.)

Answer #2: R. Yosi holds that [the duration of] Bein ha'Shemashos is like an eye's blink, and a person cannot know [when it is. He must start earlier, when it is Vadai day.]

Suggestion: Perhaps he means that when he ended his meal, this is like accepting the fast [during the day] on Shabbos!

Rejection #1: One may not accept a fast on Shabbos.

Rejection #2: We say that even Yom Kipur, for which the Tosefes (addition to the fast) is mid'Oraisa, one may eat, even if he already ended his meal, like it says in Eichah Rabsi;

Citation (Eichah Rabsi): R. Yehudah ben Beseira went to Netzivin on Erev Yom Kipur. The head of the Beis ha'Keneses invited him [to eat]. He responded "I already ate and finished";

Citation (cont.): [The head of the Keneses] said "accede to me, lest people say that a great person did not accede to me." [R. Yehudah] went with him. It concludes there that he ate one piece from every ring [of pressed figs], one piece from every cooked dish, and one cup from every barrel.

Inference: Rabanan who argue with [R. Gamliel] hold that we even interrupt (do not fast on Rosh Chodesh, Chanukah or Purim).

Question: Below, we say "this is the opinion of R. Meir, who said in the name of R. Gamliel, but Chachamim say that he fasts and completes." This shows that just the contrary, Rabanan hold that he fasts and completes [the fast]!

Answer: Perhaps the Chachamim who argue with R. Meir hold that we complete, and here it says that [R. Gamliel] admits to the Rabanan who argue with him [that we do not complete], and they hold that we interrupt.

Correction: The text says "Sen'ah", and not Sen'av, for so it is written in Ezra. He was called "ben Binyamin" for this was his Shevet. (There is no reason to say that his father's name was Binyamin);

[This family] is listed among [those who used to bring] wood for the woodpile (on the Mizbe'ach) in Ta'anis (26a), and all those listed there - Bnei Arach ben Yehudah, Bnei David ben Yehudah... all are called by the name of their Shevet.

Implied question: R. Eliezer ben Tzadok was a Kohen, like is proven in Bechoros (36a) regarding R. Eliezer bar Tzadok had a Bechor (and he asked whether or not he, a Kohen Chaver, is suspected to blemish it. If so, he is not from Shevet Binyamin!)

Answer: Perhaps his mother was [from Sen'ah,] from Binyamin, or he was the father-in-law (of Bnei Sen'ah), and he was with them to help them [bring wood].

Implied question: It says in Rosh Hashanah (19a) that days written in Megilas Ta'anis (a list of days on which miracles happened for Yisrael; one may not fast on them, and for some, even to eulogize on them), before them and after them are forbidden!

Also, a Mishnah in Ta'anis (16b) says that all days written in Megilas Ta'anis that we may not eulogize on them, before them is forbidden and after them is permitted. The 10th of Av is among the days on which one may not eulogize;

Inference: Also a Yom Tov mid'Rabanan, it is forbidden to fast the day before it!

Answer: That is only for an optional fast. An obligatory fast, like Tish'ah b'Av, we fast.

Question: How did we resolve above from R. Yosi, who discusses an obligatory fast, to Talmidim in an optional fast? It connotes that they were in an optional fast.

Strengthening of question: Also the She'altos connotes that we discuss all fasts, and the custom is that we complete even an optional fast!

Strengthening of question: Also in the Yerushalmi, we say in the name of Rav Huna that even an individual who decrees a fast on himself on Erev Shabbos, he fasts and completes it!

Answer: The proof above was proper. Since R. Yosi says that he fasts and completes, even though the fast of Tish'ah b'Av does not override Shabbos, this shows that entering Shabbos amidst affliction is not considered like fasting on Shabbos;

If so, also in an optional fast, he may complete.

Distinction: The day before a Yom Tov mid'Rabanan, we can distinguish between Tish'ah b'Av and an optional fast;

The Isur to fast the day before a Yom Tov mid'Rabanan is not because it is like fasting on Yom Tov, for also the day after is forbidden, even though [through fasting the next day] he does not fast on Yom Tov!

Rather, it is because they strengthened Yom Tov mid'Rabanan, lest people come to fast on Yom Tov itself. They decreed about an optional fast, but not for Tish'ah b'Av.

Explanation: If he wants to complete, he may complete. The question was whether or not he is permitted to complete.

Comment: My Rebbi told me in the name of R. Yedidyah that even R. Yosi argues only to say that one may complete. However, R. Yosi agrees that if he does not want to complete, this is permitted. R. Yehudah forbids to complete.

Rejection: R. Yosi said "don't you admit to me about Tish'ah b'Av that fell on Sunday, that he begins [fasting] during the day [on Shabbos]?" There, you are forced to say that it means that he is obligated, similar to here, Erev Shabbos, that he says that he is obligated to complete! (Maharshal, citing Tosfos Shantz)

Implied question: There is also Keshafim (witchcraft), which [is like an acronym for] MachCHiSHim FaMalia Shel Ma'alah (weakens the Heavenly entourage)!

Answer: This is included in Ru'ach Ra'ah.

Implied question: [There is also] the Yetzer ha'Ra! It connotes in Mo'ed Katan (17a) that sometimes a person's Yetzer ha'Ra overpowers him, and he cannot suppress it!

Answer: In any case, a person can be careful not to come to such a state.

Explanation #1 (R. Chananel): This is the yoke of the Nochri kingdom, like it says in Pirkei Avos (2:3) "beware of Reshus."

Explanation #2 (Rashi): It is a creditor.

Implied question: [When we (the Gemara) corrected Rav Nachman's teaching to say that] if he returned [to his Techum] willingly [he has only four Amos, we asked that we already learn this from our Mishnah.] Why didn't we answer that the Mishnah teaches li'Tzedadin? (If Nochrim took him out, and he is still outside, he has only four Amos. If he left willingly and Nochrim returned him, it is as if he did not leave. Rav Nachman teaches about if Nochrim took him out and he returned willingly. Our Mishnah did not teach this!)

Also in the end (we correct his teaching to say that if he left willingly and Nochrim) returned him (he has only four Amos), we need to say that really, [it is a Chidush, for one might have thought that the Mishnah] teaches li'Tzedadin!

Answer: He prefers to answer in a way that the first [version of the] words of Rav Nachman will be fulfilled. (The first version was "if one left the Techum willingly, he has only four Amos." We conclude like this, that he left willingly, just we add that Rav Nachman discusses when Nochrim returned him.)

Implied question: What was brought from outside the Techum is forbidden to the one to whom it was sent! (Also here we should forbid!)

Answer: That is when [what was brought] is outside [its] Techum. Here, that now the Peros are in their [initial] place, they are permitted even to the one who took them out.

Implied question: If one cooked on Shabbos b'Mezid, he may not eat it! (We should say similarly here!)

Answer: There is different, for he did an Isur Torah.

Inference: No person may eat them.

Implied question: If something was brought [from outside the Techum] for a Yisrael, it is permitted to a different Yisrael!

Answer: Here that an Isur was done through a Yisrael, [Chachamim] were stringent. (They permit to another Yisrael only when a Nochri brought them.)