21 Taanit 12a

The halakha is that which you said, that one may fast for a few hours, provided that one took a fast of a few hours upon himself, and that he fasted and did not taste anything until the evening. Abaye said to Rav Ḥisda: This ruling is obvious, since it is a full-fledged fast, as one ultimately fasts the entire day. Rav Ḥisda answered: No, it is necessary to say this halakha in a case where he changed his mind, i.e., he began the day without intending to fast, but for various reasons he did not eat, and halfway through the day he decided to continue fasting for another few hours until nightfall. Rav Ḥisda maintains that this kind of fasting for hours is considered a fast. And Rav Ḥisda said: Any fast upon which the sun does not set, i.e., when one eats in the middle of the day, it is not called a fast at all. The Gemara raises an objection: The members of the priestly watch, the priests and Levites who are serving in the Temple that week, fast on a communal fast, like the members of the non-priestly watch who are attached to specific groups of priests, but they do not complete the fast with the rest of the community. This indicates that even a fast lasting only a few hours is called a fast. The Gemara explains: There they abstained from food merely to cause themselves distress, as an act of solidarity with the rest of the community, but this was not considered a full-fledged fast. The Gemara cites another proof: Come and hear, as Rabbi Elazar, son of Rabbi Tzadok, said: I am a descendant of Senaah, son of the tribe of Benjamin, and once the Ninth of Av occurred on Shabbat, and we postponed the fast until after Shabbat, as Shabbat supersedes the fast of the Ninth of Av. And we fasted on that day, but we did not complete the fast, due to the fact that the tenth of Av is a holiday of ours, a private holiday for our family (26a), and one does not fast on holidays. This proves that a fast of only a few hours is nevertheless called a fast. The Gemara rejects this claim: There too, they did so merely to cause themselves distress. The Gemara again attempts to disprove the opinion of Rav Ḥisda, this time by citing a custom of Rabbi Yoḥanan: Come and hear, as Rabbi Yoḥanan occasionally said: I shall be in observance of a fast until I come to my house. This indicates that one can take a fast upon himself even for just a few hours. The Gemara rejects this proof as well: There he did this to excuse himself from the household of the Nasi. At times, Rabbi Yoḥanan received an invitation to dine at the house of the Nasi, but wanted to decline. To avoid insulting the Nasi, he would say that he had taken a fast upon himself, and afterward he would eat at home. Since this was not a real fast, it is not proof of the legitimacy of a partial-day fast. Shmuel said: Any fast that one did not take upon himself while it was still day is not called a fast. The Gemara asks: And if one happened to sit in observance of a fast that day, what is that considered? Rabba bar Sheila said: He is likened to a bellows that is full of air. His behavior does not constitute a fast, as he is merely a container full of air, without food. § The Gemara asks: When does one take a fast upon himself? Rav said: One takes a fast upon himself in the afternoon of the day preceding the fast, from midday onward. And Shmuel said: One must take a fast upon himself for the following day at the end of the afternoon prayer. Rav Yosef said: It stands to reason that the halakha should be in accordance with the opinion of Shmuel, as it is written in Megillat Ta’anit, after a list of all of the Festivals established by the Sages in commemoration of various events throughout Jewish history: Therefore, whoever has bound himself beforehand with an obligation to fast on one of these commemorative days, he will be bound. The Gemara explains the apparent proof from the text of Megillat Ta’anit. What, is it not the case that this means that he binds himself to the obligation at the time of prayer, and if he did not do so at that point in time his fast does not go into effect? The Gemara rejects this explanation: No; the text should be amended by one letter so that it does not read yeisar, he shall bind himself, but yei’aser, he shall render himself prohibited from transgressing these instructions. In other words, the text is explaining that despite a prior vow to fast, it is prohibited for one to do so on the commemorative days enumerated in Megillat Ta’anit. The Gemara comments: Rabbi Ḥiyya and Rabbi Shimon, son of Rabbi Yehuda HaNasi, disagree with regard to this dispute. One of them said that the text of Megillat Ta’anit reads: He will be bound, and the other one said that it reads: He will be prohibited. The one who said: He will be bound, this is as we just say. However, according to the one who said: He will be prohibited, what does it mean? The Gemara explains: This is as it is taught in Megillat Ta’anit: Whoever has taken upon himself beforehand to fast on one of these Festivals days, he will be prohibited to eat and drink. How so? With regard to an individual who took upon himself to fast on each Monday, Thursday, and Monday series of the entire year, and the commemorative holidays written in Megillat Ta’anit occurred on these days, if his vow preceded our decree, i.e., the decree of the Sages establishing these commemorative days, his vow annuls our decree and he must fast, but if our decree preceded his vow, our decree annuls his vow and he may not fast on those days. The Sages taught in a baraita: Until when may one eat and drink on communal fasts, when one fasts during the day but not the preceding evening? Until dawn. This is the statement of Rabbi Yehuda HaNasi. Rabbi Elazar, son of Rabbi Shimon, says: Until the call of the rooster, which is before dawn. Abaye said: They taught this ruling, that one may eat all night, only if he has not finished his evening meal, as he may continue eating the same meal all night. However, if he has finished his meal, he may not eat any more. Rava raised an objection to Abaye from a baraita: If one finished his meal and stood up, nevertheless, he may eat more. This shows that one may in fact eat throughout the night, even if he has finished his meal. Abaye answered: There the baraita is referring to a situation where he has not yet removed or cleared the table, and therefore it is as though he has not yet finished his meal. Some say a slightly different version of this discussion. Rava said: They taught this ruling only if he did not sleep after eating, but if he slept he may not eat anything else that night. Abaye raised an objection to Rava from a baraita: If one slept and arose from his sleep during the night, he may eat. Rava explained: There the baraita is referring to one who was merely dozing, and was not fully asleep. The Gemara asks: What are the circumstances of dozing? Rav Ashi said: