Can you fast for half a day?

Question

Is there any story in the torah or talmud about, half a day fast?

Like from morning until dinner?

Answer 1

Can you fast half a day? 10 Tevet on a Friday by Rabbi Ethan Tucker goes into detail on the subject. He first discusses a fast that occurred on Friday and whether one must fast to the end even though it is already Shabbos. He discusses this in terms of Asarah B'Teves and private fasts as those are the only ones that can occur under our fixed calendar.

He then discusses the gemaras about someone deliberately fasting for part of a day. It appears from the citations that I marked in bold text, that many poskim seem to say that a partial fast is not valid. Note especially the quote

Shulhan Arukh OH 562:1 affirms that a partial day fast is not ritually/liturgically meaningful.

However, Based on the Terumat Hadeshen and the Rama, he comes to the conclusion

The partial day fast has enough integrity to be referred to as ביום צום תעניתנו. This practice is affirmed by Magen Avraham, Mishnah Berurah and many other later authorities.

As always one must CYLOR for a specific psak in particular cases.

Indeed, we have a few examples of fasts that were truncated earlier in the day. Mishnah Ta'anit 2:6 speaks about the behavior of the priests on duty in the Temple when fasts were being declared for lack of rain. We are told that during the initial two sets of three fasts, at least some of the priests on duty only fasted a partial day. Tosefta Ta'anit 3:6 features R. Elazar bR. Tzadok reporting that his family was descended from those who performed an important Temple service on the 10th of Av, making this a joyous day for them. When 9 Av fell on Shabbat, such that the fast was pushed to Sunday, his family would only fast a partial day. These both seem to be cases of fasts that were begun in the morning (or the night before) and then ended early midway through the day.

The Talmudim then take up an even more targeted category of תענית שעות-a fast that only lasts a set number of hours.

Here is the summary of what he found regarding private fasts that are supposed to

On Bavli Ta'anit 12a, Shmuel makes a general point about the need for any optional fast to be explicitly accepted at Minhah the day prior. It is not clear if he believes this to apply to a תענית שעות as well.

On Bavli Ta'anit 12a, R. Hisda says that any fast that does not run through the end of the day (sunset) is not a valid fast. The Talmud then cites the cases of the priests and R. Elazar bR. Tzadok's family; in both cases, people did end their fast before the end of the day. The Talmud deflects these practices as being devoid of the status of a real fast; essentially, these people electively decided to adopt an ascetic practice, but it has no liturgical/ritual meaning to it. The Talmud also cites a version of the tradition about R. Yohanan in the Yerushalmi, where he commits to a fast until he arrives home from the Patriarch's house, which he was visiting or was summoned to. That case is dismissed as R. Yohanan making that public pronouncement so he could extricate himself from the Patriarch's presence.

Finally, there is a statement by R. Hisda that appears in two different versions and is read differently by different commentators. Rambam understands his ruling to invalidate a fast that is ended early (before the end of the day), but consistent with beginning a fast midway through the day (like R. Yonah's practice in the Yerushalmi). In other words, a private fast must always end at sunset/nightfall, but it can begin whenever one wants, as long as it is planned out that way in advance. This reads R. Hisda's two statements as making the same legal point, in different words and in different textual contexts. Rashi and Ra'avad read R. Hisda as forbidding beginning a fast late, just as his prior statement forbade ending it early. In other words, for Rashi and Ra'avad, the collective effect of R. Hisda is to eliminate the ritual/liturgical possibility of a half-day fast. The only meaning of תענית שעות is that a person who fasted an entire day but only did so with intention starting part way through can get credit for a full-day of fasting and have it be ritually/liturgically meaningful. **Both Rambam and Ra'avad follow the Bavli's seeming conclusion that ending a fast early does not have ritual/liturgical meaning.

Shulhan Arukh OH 562:1 affirms that a partial day fast is not ritually/liturgically meaningful.**

Terumat Hadeshen #157, however, reports that the practice in his community is for grooms to fast a partial day when a wedding is held on Friday afternoon, and argues that this practice is consistent with the permission for תענית שעות, without fully making his case. Rema settles on a kind of compromise position: 1) A part-day fast by a community cannot trigger changes in communal liturgy, such as the added berakhah in the public Amidah. 2) One may commit to fasting part of the day and add Aneinu into the private Amidah, since this is added into שמע קולנו, the blessing that is a fitting receptacle for all private requests. The partial day fast has enough integrity to be referred to as ביום צום תעניתנו. This practice is affirmed by Magen Avraham, Mishnah Berurah and many other later authorities.

In sum, one taking on an optional fast day (which includes the case of one who chooses to fast on public fast days that s/he believes are optional today), can choose to fast until Minhah, insert Aneinu into Minhah and then break the fast. But no changes should be made to the public Amidah in Minhah in that situation, even if the entire community is fasting a partial day.

Note that Taanis 12 says about a ta'anis because of a dream that one fasts the day after the dream even though it was not originally planned. However, that is a full day fast.

תענית יב”

Fasting for a bad dream

רבה בר מחסטא אמר רב חמא בר גוריא אמר רב יפה תענית ואמרלחלו כא לנעורת ואמר רב חסדא ובו ביו וא”ר יוס ואפילו בשבת

And Rabbah bar Mechasya said in the name of R’ Chama bar Gurya who said in the name of Rav a fast is helpful [to nullify the harmful message] of a dream as fire is [effective to burning] flax. And R’ Chisda said [that it must be observed] that day and R’ Yosef said [that it may be observed] even on Shabbos.

The Tur 1 cites opinions who assert that nowadays people should not fast on Shabbos if they experience a bad dream because we no longer know how to properly interpret dreams to know whether the dream portends good or bad things to come. Rav Schneur Zalman of Liadi 2, the Shulchan Aruch HaRav, explains that this applies even to those dreams that were explained by Chazal. Just like medical treatments men-tioned by Chazal are no longer practiced, so too their interpretation of dreams is no longer practiced. Therefore, one should not fast on Shabbos since we are uncertain whether the dream is truly bad. Rav Yaakov Chaim Sofer 3, the Kaf HaChaim, cites opinions who advise that rather than fasting on Shabbos following a bad dream one should refrain from talking with oth-ers and recite the entire book of Tehillim. On Sunday if they wish to fast they may fast and the appropriate tzedakah should be given at that time.

Answer 2

The Ba'al HaTanya writes in Igeret HaTeshuvah chapter 3 concerning breaking up personal fasts into half days. He cites the Yerushalmi that fasting until chatzot is still considered a fast. This can be found in Nedarim 8:1, Gemara beginning with "Rabbi Acha" and it continues with a few examples of rabbis taking on partial fasts.