Reference: I explained this in Avodah Zarah (34a), and also a white garment without a cuff [in which Moshe served during the Milu'im].

[I explained there also] the entire ruling of Ta'anis Sha'os, that he must accept [before] the night [before the fast] in Tefilas Minchah, in Shome'a Tefilah. If it (the day before) is Shabbos, he accepts in Elokai Netzor;

He must say "tomorrow I will have an individual fast", so it cannot be confused with a Ta'anis Tzibur.

Implied question: We say (Rosh Hashanah 18b) that after Churban Beis ha'Mikdash, Megilas Ta'anis was Batel;

The episode with R. Eliezer bar Tzadok was after the Churban, for if it was before the Churban, we say that since there is Shalom, there is no fast!

Answer: (It was after the Churban.) This Tana holds that Megilas Ta'anis was not Batel.

Explanation: Ye'aser with an Aleph is from the root "Asur". He forbade himself;

"Yei'sar" is from the root "Hasarah" (veering). He will veer from the vow and be permitted to eat tomorrow, for it will be Yom Tov, and one does not fast.

Rashi's opinion: His vow is totally pushed off. He does not complete the fast (fast another day to compensate).

Ri's opinion: The Ri was unsure about this.

Conclusion: One should be stringent. Only that day he does not fast, but he should complete the fast the next day.

Similarly, one should be careful not to fast on Rosh Chodesh, Chanukah and Purim. Regarding these, surely Megilas Ta'anis is not Batel.

Opinion #1 (Tosfos' Rebbi): "Nim v'Lo Nishmat" (he is sleeping, but not fully sleeping) at the beginning of sleep. He is Tir v'Lo Tir (he is not fully awake) at the end of sleep.

Question: Pesachim 120b connotes unlike this. We say that if (a group eating Korban Pesach) Nisnamnu (dozed), they may continue eating, but not if Nirdemu (they slept);

Citation (120b - Rav Ashi): Nisnamnu is Nim v'Lo Nim...

There we discuss the beginning of sleep, both regarding Nim v'Lo Nim and Tir v'Lo Tir!

Opinion #2 (Tosfos): Rather, I explain like Rashi. Had it said only Nim v'Lo Nim, one might have thought that Tir is more (asleep) than Nim;

Had it said only Tir v'Lo Tir, one might have thought that Nim is more (asleep) than Tir;

Therefore, it needed to say both. All of them are at the beginning of sleep, like it explains there (120b).

Citation (Yerushalmi, regarding this Beraisa): Until when may one eat and drink? Until Kros ha'Gever. The same applies until dawn. This is when he stipulated. If he did not stipulate, and he slept, he may not eat.

Explanation: This is a shoe without a sole, Sola in old French.

Explanation: This is why Shmuel said that he borrows his fast and pays. It is only if he cannot pain himself. If he can pain himself, he may not borrow his fast.

This is the conclusion. If he said "I am in a fast tomorrow", he must pain himself. He may not borrow without need. Even if he ate b'Shogeg, he must complete [the day without eating more].

Opinion #2: The Ri was unsure whether or not one may borrow and repay if he already accepted to fast, and he fasted until half or a quarter of the day, even if he cannot pain himself;

But if he did not (begin to) fast he certainly may borrow another instead, or if he mistakenly ate he is permitted to eat and make it up on another day;

If he fasted on Shabbos, e.g. for Ta'anis Chalom, for which one must pay (fast another day), like it says "just he must fast [another day] for his fast" -

If that day [after Shabbos] is Rosh Chodesh, he postpones his [compensation] fast until another day in the week. My Rebbi, she'Yichyeh, agreed.

Inference: We do not [forbid Melachah at night, just like elders do not gather] at night.

Question: Why do we eat and drink [only] during the day [before the fast]? Also the entire night they may eat and drink!

Just like elders gather during the day, also fasting is during the day, but not at night!

Answer: We learn fasting from gathering only regarding doing Melachah. Only Melachah is learned from gathering, for the verse says only "gather elders and sanctify a fast";

The verse does not discuss eating.