The Mitzvah of Personal Fasts and Set Fast Days
It is a mitzvah, for every Jewish community, when they are beset with trouble, God forbid, to fast and pray to Hashem, blessed is His Name, (for rescue) from their troubles. But if the time is not appropriate for fasting, for instance, if people are fleeing, they are not permitted to fast, lest their strength will be sapped. However they should resolve to fast a certain number of fast days, when they are saved. This will be counted as if they were now fasting, as we find concerning Daniel, about whom it is written, "He said to me, 'Do not be afraid, Daniel; for from the day you resolved to understand and to fast before your God, your words were heard'" (Daniel 10:12).
ת"ר לפי שראה אדם הראשון יום שמתמעט והולך אמר אוי לי שמא בשביל שסרחתי עולם חשוך בעדי וחוזר לתוהו ובוהו וזו היא מיתה שנקנסה עלי מן השמים עמד וישב ח' ימים בתענית [ובתפלה]
With regard to the dates of these festivals, the Sages taught: When Adam the first man saw that the day was progressively diminishing, as the days become shorter from the autumnal equinox until the winter solstice, he did not yet know that this is a normal phenomenon, and therefore he said: Woe is me; perhaps because I sinned the world is becoming dark around me and will ultimately return to the primordial state of chaos and disorder. And this is the death that was sentenced upon me from Heaven, as it is written: “And to dust shall you return” (Genesis 3:19). He arose and spent eight days in fasting and in prayer.
If the seventeenth of Mar-Cheshvan has come without the rain having yet descended, private individuals commence to keep three fast-days, on the preceding nights of which it is lawful to eat and drink, to work [on the fast-day], to bathe, to anoint the body, to wear [leather] shoes, and to perform the marriage duty.
Aneinu: From Peninei Halakhah - Yeshivat Har Brakha
The Rabbis prescribed that we add a special blessing for the fast, called Aneinu, in our prayers. The cantor inserts it in between the blessings of Go’el Yisrael and Refa’einu when he repeats the Shemoneh Esrei of Shacharit and Minchah. He says it only if there are at least six people in the congregation fasting, and he has to be one of them (SA 566:5).[13]
Individuals, however, do not say Aneinu as a separate blessing in their silent prayers. Rather, they insert it in the middle of the blessing of Shomei’a Tefillah (Ta’anit 13b). There are various customs as to when we say Aneinu. Some say that one should recite Aneinu in all three prayers of the day. And even though we do not fast at night, one should say it in Ma’ariv because the day as a whole is called a fast day. Yemenite Jews and some Sefardic Jews follow this custom. Most Sefardim say Aneinu only when the fast is in effect. Therefore, on the minor fasts they say it in Shacharit and Minchah, and on Tish’a B’Av, they say it also in Ma’ariv (based on Razah, KHC 565:17). Ashkenazi Jews are accustomed to saying Aneinu in Minchah alone, because they are concerned that perhaps someone will say it in Shacharit, become weak during the day, and break his fast. Then, his statement “on this day of our fast” will turn out to be a lie. Therefore, they say Aneinu only in Minchah, because one who has fasted this long will probably complete the fast (based on the Geonim and Rashi; Rama 565:3). Everyone should continue his family custom.
One who eats less than an olive-sized portion of food or drinks less than a cheek full of liquid is considered to still be fasting and should say Aneinu. But if one eats or drinks more than that, he has broken his fast and may not recite Aneinu.[14] (Nonetheless, he must continue fasting, as we mentioned above in sec. 6).
[13]. The Shulchan Aruch (566:3) writes that the cantor says Aneinu only if there are ten people fasting. However, many Acharonim explain that this is true only if a particular community establishes a day of fasting, but regarding the four fasts that the Prophets instituted, six fasters are sufficient. See MB 566:14, Torat HaMo’adim 3:12, and Piskei Teshuvot 566:4. When there are less than six people fasting, the cantor should say Aneinu in the blessing of Shomei’a Tefillah, as an individual does.
If the cantor forgets to say Aneinu and has not yet said God’s name at the end of the blessing of Refa’einu, he should go back and recite Aneinu [in its proper place]. If, however, he has already said God’s name, he should say Aneinu in the blessing of Shomei’a Tefillah, as an individual does. And if he forgets to insert it even there, he should say it, without its conclusion [“Blessed are You, O Lord, Who answers in time of trouble”], after “Blessed are You…Who blesses His nation Israel with peace,” before he says, “May the expression of my mouth …” (MB 119:19). An individual who forgets to say Aneinu in Shomei’a Tefillahshould insert it in the supplications that follow the Shemoneh Esrei. See Piskei Teshuvot 565:2.
[14]. There are varying opinions on this issue, and the [Chafetz Chayim] writes in his Mishnah Berurah (568:3), citing Nahar Shalom, that one should say Aneinu. In his Bi’ur Halachah (565:1), however, he cites Ma’amar Mordechai as saying that one who eats on a fast day should not say Aneinu at all. The author of Shevet HaLevi (5:60) reconciles this contradiction by saying that one who is exempt from fasting should not say Aneinu, while one who eats accidentally should say it, because the fast is [still] relevant to him. In my humble opinion, however, one can clearly infer from the Ashkenazi custom not to say Aneinu in Shacharit because one may not fast [the entire day] that one who has already eaten should not say it. Many Sefardim adopt this custom, as Torat HaMo’adim (1:16) writes. Even one who [merely] intends on eating, but has not yet done so, should not say Aneinu (SA 562:1; see MB ibid. 6).
It is also unclear how much one needs to eat in order to be considered no longer fasting. Regarding Yom Kippur, Chazal say that eating food the size of a dried date in the time it takes to eat a p’ras (half a loaf), or drinking a cheek full of liquid, nullifies the pangs of fasting. However, the Shulchan Aruch (568:1) writes that one who eats an olive-sized portion of food in the time it takes to eat a p’ras has broken his fast, for an olive-size is the standard measure for all [mitzvot or sins related] to eating. See Piskei Teshuvot 568:1, who explains that the Acharonim debate the matter. And since there is doubt regarding this issue, it is better to be passive and not say Aneinu if one ate an olive-size of food.
There are days that all the people of Israel observe as fasts on account of the tragic events which occurred on them, the purpose being to appeal to the hearts and to lay open the paths of repentance. This serves as a reminder of our evil doings, and the deeds of our fathers which were like ours now, resulting in the afflictions endured by them and by us. By remembering these things we are likely to repent and do right, as it is written: "They shall confess their sins and the sins of their fathers…" (Leviticus 26:40). They are as follows: The third day of Tishri, on which Gedaliah the son of Ahikam was slain, the last ember of Judea's independence was extinguished, and her dispersion was made complete. The tenth of Teveth, on which wicked Nebuchadnezzar, king of Babylon, pressed on Jerusalem and placed it under siege and stress. The seventeenth day of Tammuz, on which five things occurred: the tablets were broken, the daily offering was discontinued before the destruction of the first Temple, the walls of Jerusalem were breached before the destruction of the second Temple, wicked Apostomos burned the Torah and set up an idol in the Temple. The ninth of Av, on which five things happened: it was decreed in the wilderness that the people of Israel were not to enter the promised land; the Temple was destroyed both the first time and the second time, and a great city named Betar was captured. It was inhabited by tens of thousands of Jews. They had a great ruler, thought by all of them, including great scholars, to be King Messiah. But he fell into the hands of [the Roman] pagans, who killed them all, a calamity as great as the destruction of the Temple. On that day, the ninth of Av, predestined for Israel's reverses, the wicked Turnus Rufus plowed up the Temple site and its surroundings, in fulfillment of the prophetic utterance: "Zion shall be plowed up like a field" (Jeremiah 26:18).
As Rav Ḥana bar Bizna said that Rabbi Shimon Ḥasida said: What is the meaning of that which is written: “Thus said the Lord of hosts: The fast of the fourth month, and the fast of the fifth, and fast of the seventh, and the fast of the tenth, shall become times of joy and gladness, and cheerful seasons, to the house of Judah” (Zechariah 8:19). It calls them days of “fast” and it calls them “times of joy and gladness.” How so? When there is peace in the world, they will be times of joy and gladness, on which eulogies and fasting are forbidden; but when there is no peace, they are days of fasting. Rav Pappa said that this is what it is saying: When there is peace in the world and the Temple is standing, these days will be times of joy and gladness; when there is persecution and troubles for the Jewish people, they are days of fasting; and when there is no persecution but still no peace, neither particular troubles nor consolation for Israel, the halakha is as follows: If people wish, they fast, and if they wish, they do not fast.
Who is obligated to fast on the set fast days?
הכל חייבים להתענות ארבע צומות הללו ואסור לפרוץ גדר: הגה מיהו עוברות ומיניקות שמצטערות הרבה אין להתענות (הגהות מיימוני והמגיד פ"ה) ואפי' אינן מצטערות אין מחוייבות להתענות אלא שנהגו להחמיר ודוקא בג' צומות אבל בט' באב מחויבות להשלים (כדלקמן סי' תקנ"ד סעיף ה'):
We are all obliged to fast these four fasts and it is forbidden to breach the fence. Gloss: However, women who are pregnant or nursing and would suffer a lot do not need to fast ... and even if they would not suffer they are not obliged to fast, unless they are generally strict [as regards fasts]. This [ruling] applied to the three minor fasts, but on Tish'ah Be'Av, they must complete the fast (see below in Section 554, paragraph 5).
Prayer on Fast Days
MISHNA: The shofar that was used on Rosh HaShana in the Temple was made from the straight horn of an ibex, and its mouth, the mouthpiece into which one blows, was plated with gold. And there were two trumpets, one on each of the two sides of the person sounding the shofar. The shofar would sound a long blast, whereas the trumpets would sound a short blast, because the mitzva of the day is with the shofar.
And in contrast, the shofarot used on public fast days were made from the curved horns of rams, and their mouths were plated with silver. There were two trumpets in the middle between the shofarot, and the shofar would sound a short blast, whereas the trumpets would sound a long blast, for the mitzva of the day is with the trumpets.
Fasting and Society
“Why, when we fasted, did You not see? When we starved our bodies, did You pay no heed?” Because on your fast day You see to your business And oppress all your laborers! Because you fast in strife and contention, And you strike with a wicked fist! Your fasting today is not such As to make your voice heard on high. Is such the fast I desire, A day for men to starve their bodies? Is it bowing the head like a bulrush And lying in sackcloth and ashes? Do you call that a fast, A day when the LORD is favorable? No, this is the fast I desire: To unlock fetters of wickedness, And untie the cords of the yoke To let the oppressed go free; To break off every yoke. It is to share your bread with the hungry, And to take the wretched poor into your home; When you see the naked, to clothe him, And not to ignore your own kin. Then shall your light burst through like the dawn And your healing spring up quickly; Your Vindicator shall march before you, The Presence of the LORD shall be your rear guard.