Just as it is a mitzvah for the community to fast and to pray in times of trouble, so it is a mitzvah for every individual, if, God forbid, any trouble befalls him, for example, a member of his family is sick, or he lost his way, or he is confined in prison on false charges, to fast and to pray to God, and to plead for mercy from Him, may His Name be blessed, to deliver him. Suffering is one of the ways that leads to repentance. A person should not say, his troubles just happened by chance, for it is said: (Leviticus 26:23,24). "If you treat My (acts) as chance, then I will treat you with the fury of chance"; which means, when I bring distress on you to cause you to repent and you will say this happened by chance, then I will add to your (suffering) the fury of the same "chance." Man must know that because of his sins, God brought on him all this trouble. He should, therefore, examine his deeds and return to God, and He will have mercy on him.

If you wish to fast, you must make a commitment to do so¹ 562:23) on the preceding day, during the Shemoneh esrei of Minchah.² (nine and one half hours after sunrise), but, if not, you can do so at an earlier Minchah. (Ibid. 562:27) In the berachah Shema koleinu [Hear our voices], you should mentally accept upon yourself, to fast and before saying Yiheyu leratzon, you should say, Master of the universe, I come before You to accept a fast etc.³ 562:6; Mishnah Berurah 562:30,31) (as it is written in the siddur [prayer book].) Even though you eat and drink, afterwards until daybreak, it does not matter. If you want to fast several days in succession, although you will eat and drink during the intervening nights, one acceptance [commitment] is sufficient for all. But if you accept upon yourself [to fast] several days that are not in succession, like Monday, Thursday, and Monday, you should make a separate commitment, for each day you fast, in the Minchah preceding it.⁴ 562:8. The prevailing custom is, however, that one acceptance is sufficient.

footnotes: ¹ If you do not make this commitment, it is not considered a fast. Thus, if you are fasting to fulfill a vow, and you fasted without making a prior commitment, you have not fulfilled your vow to fast. (see Mishnah Berurah ² It is preferable to do so at a Minchah Ketanah Shemoneh Esrei ³ If you forgot to say this prayer, but you did, however, make a mental commitment to fast, it is sufficient. (Shulchan Aruch ⁴ See Ramah

If you are accustomed to fast⁵ 562:2) during the Aseres yemei teshuvah, [the Ten Days of Penitence] or on the first day of Selichos or on erev Rosh Hashanah, you do not have to make a formal commitment because they are accepted on the basis of the prevailing custom. Similarly, fasting because of a disturbing dream, does not require prior acceptance; neither do the fasts on Monday, Thursday and Monday following Pesach and Sukkos. If you answer Amein after the Mi shebeirach [a blessing for those who fast on these days] and you intended to fast, this is sufficient, and no other form of acceptance is needed. Nevertheless, if you change your mind, and do not wish to fast, you may [eat], since you did not expressly commit yourself⁶, even without a verbal commitment. (See Mishnah Berurah 562:39) and did not verbally declare that you intend to fast.

footnotes: ⁵ This applies, even if this is the first time you are fasting on one of these days. So long as it is an accepted custom in your community, you may join and rely on those who have fasted. (Biyur Halachah ⁶ See Paragraph 4, and, accordingly, it would seem that it is best to be stringent, if you had in mind to fast when you answered Amein

Even if you did not commit yourself verbally, to fast; doing so, only in your mind, resolving to fast the following day, and even [if you made this resolution] not during Minchah, but either before or after Mincha, while it was still daytime, it is considered an acceptance, and you are required to fast.

When you fast, you should not indulge in enjoyment, nor act frivolously. You should not go around happy and cheerful, but rather with anxiety and sorrow,⁷. 568:50) as it is said: "Why should a living man bemoan? A man, because of his sins!" (Lamentations 3:39).

footnotes: ⁷ You should be careful to control your anger and to conduct your business activities in such manner, as not to become angry. (Ibid

On a private fast day, you are allowed to rinse your mouth with water, in the morning.

If you committed yourself to fast without being specific, you must complete the fast until the stars come out, even on erev Shabbos.⁸, however, rules, according to the Poskim, who maintain that on erev Shabbos, it is sufficient to fast until the congregation leaves the synagogue, after they accepted Shabbos (Kabbolas Shabbos). (Ibid. 249:21 and also Biyur Halachah 249:4)

footnotes: ⁸ Ramah

If someone fasts and publicizes the fact, bragging about it, he will be punished for it.⁹, however, indicates that you should deny that you are fasting, unless you are asked to join someone in a meal, in which case you have no choice but to state you are fasting. On public fast days, however, you may say you are fasting in order to encourage others to do so. (Mishnah Berurah 565:14) But if people urge him to eat, he may let them know that he is fasting.

footnotes: ⁹ However, if you are asked if you are fasting, it would seem that you are permitted to acknowledge that you are. Mishnah Berurah

If you observe a fast, even if it is a private fast, whether it is a voluntary fast, or a fast because of a disturbing dream, you should say in the Shemoneh esrei of Minchah, in the berachah Shema koleinu [Hear our voice], the prayer Aneinu [Answer us], the same as on any public fast day. (And although you are only one individual, you should say it in the plural, and not deviate from the formula, that was established by the Sages). Before saying Yiheyu leratzon, you should say Ribbon ha'olamim [Master of the universe] etc.¹⁰

footnotes: ¹⁰ These are the words of the prayer: רִבּוֹן כָּל הָעוֹלָמִים גָלוּי וְיָדוּעַ לְפָנֶיךָ שֶׁבִּזְמַן שֶׁבֵּית הַמִקְדָשׁ הָיָה קַיָם אָדָם חוֹטֶא מִקְרִיב קָרְבָּן וְאֵין מְבִיאִים מִמֶנוּ אֶלָאחֶלְבּוֹ וְדָמוֹ, וְאַתָּה בְּרַחֲמֶיךָ הָרַבִּים מְכַפֵּר . וְעַתָּה בַּעֲוֹנוֹתֵינוּ חָרַב בֵּית מִקְדָשֶׁךָ וְאֵין לָנוּ מִקְדָשׁ וְלֹּא כֹהֵן שֶׁיְכַפֵּר בַּעֲדֵינוּ, לָכֵן יְהִי רָצוֹן מִלְפָנֶיךָ שֶׁיְהֵא חֶלְבִּי וְדָמִי שֶׁנִתְמַעֵט הַיוֹם כְּחֵלֶב מוּנַח עַל הַמִזְבֵּחַ לְפָנֶיךָ וְתִרְצֵנִי.Master of the Universe, we know that during the time the Holy Temple stood, if a person sinned, he brought a sacrifice. The only part of the animal to be offered on the Altar was its blood and its fat, and in Your great compassion, You forgave him. But, now, because of our sins, the Holy Temple was destroyed, and we have no temple service to atone for us. Therefore, may it be Your will, that my blood and fat, that are diminished by fasting this day, be considered as though they were offered before You on the Altar, and, thus, may I find acceptance before you.

If you vowed to fast one day or ten days, but you did not specify the particular day or days, on which you were going to fast, but you stated it in indefinite terms, even if you committed yourself in the Minchah prayer to fast the following day, if an urgent need arises for you to eat something, for example, if you are invited to a meal, that is considered a mitzvah, even though you are not a party to the festive meal; or an important person urges you to join him in a meal, and it is difficult for you to refuse him, or if you do not feel well [enough to fast], then you may "owe" the fast [and repay it later] and you may eat on that day,¹¹ 568:9,10) even if you already began to fast, and instead of fasting that day, you should fast another day. This is permissible only if the fast, you had accepted on yourself, was meant to fulfill a vow, however, if you had made no vow, but (specifically) committed yourself during Minchah to fast the following day, then, even if it causes you great distress, you are not permitted to "owe" your fast and repay it another day.

footnotes: ¹¹ You should, however, fast a part of the day, and it is sufficient, therefore, if you eat at a later hour than you are accustomed; for example, if you usually eat at 10 A.M., you should eat at 11 A.M. In this way, at least, some of your vow will be fulfilled on the day you selected. (Mishnah Berurah

If at the time you made the vow, you specified certain days, and you also accepted it on yourself during Minchah, you may no longer "owe" your fast (to be paid later.).

If you undertook to fast, and are experiencing grave discomfort because of it, you may redeem it with money, according to your means, and you should give the money to the poor. But for a fast in fulfillment of a vow, redemption is of no avail. On a fast day, decreed by the community, redemption is also of no avail, unless the community made such a stipulation.

If you made a vow to fast on Monday, Thursday and Monday, you may change it to fasting on Thursday, Monday and Thursday, but not to any other days (of the week), for it is presumed, that you had these days in mind, because they are the days when the court is in session.

If you fast on Monday, Thursday, and Monday following Pesach and Sukkos, as well as on the Aseres yemei teshuvah, [the Ten Days of Penitence], and you did not commit yourself to fast during Minchah, but your fasting is motivated by the custom, even if you had in mind to fast, when responding Amein to the Mi shebeirach, as long as you did not resolve to fast during Minchah, then, if there is a bris [circumcision] or a pidyon haben [redemption of first born male] or any other meal celebrating a mitzvah, you are fulfilling a mitzvah by eating,¹². 568:17) and you need not absolve your vows, for whoever fasts on these days is following the rules imposed by custom, and this custom of fasting was not intended in regard to a meal in celebration of a mitzvah.

footnotes: ¹² In such a case, you need not even repay the fast on a later day. (Ibid

In cases, where you are permitted to eat at a meal celebrating a mitzvah, the fast is, thereby, ended altogether, and you may eat afterwards, even in your own house; but before the festive meal, it is forbidden to eat. Only the father of the infant on the day of the bris and the sandak may eat even before the meal, since to them it is like a Yom Tov.

However, if you ate on a fast day in violation of the law, whether inadvertently or intentionally, you must complete the fast, even after having eaten. Afterwards, you must fast on Monday, Thursday and Monday, as an atonement for eating on the fast day.¹³ of food, but if you ate less than that, you need not fast another day as an atonement. Similarly, in regard to drinking, if you drank less than a cheekful, you may continue the fast, and it is not required that you fast on another day. (Ibid. 568:5) ¹⁴ makes no mention of the need to fast on Monday, Thursday and Monday as an atonement. It would seem that one day is sufficient as an atonement. It is certainly true that you must fulfil your vow at a later time if you had to fast because of a vow you had made.

footnotes: ¹³ This rule applies only if you ate a kazayis ¹⁴ Mishnah Berurah

If you fast because of personal trouble, and the trouble passed, or if you fast for a sick person, and that person recovered or died, you must complete all the fast days that you committed yourself to observe. If someone accepted upon himself to fast or to do some other mitzvah until his son becomes Bar Mitzvah,¹⁵. (See Ramah, Yoreh Deiah 220:15) and the son died before that time, he must, nevertheless, fulfill his vow until the time his son would have become Bar Mitzvah. However, if you find out that before you committed yourself to fast, the cause for fasting had already passed¹⁶ 569:7) and your commitment, thus, was made in error, then you are not required to complete it.

footnotes: ¹⁵ This rule is not mentioned in Mishnah Berurah ¹⁶ However, if the cause for fasting passed after you made the commitment, even though you had not actually started the fast, you must, nevertheless, complete the fast. (Mishnah Berurah

Fasting is beneficial when accompained with repentance for nullifying the portents of a bad dream,¹⁷. 220:5) just as fire consumes the refuse of flax; but only if [you fast] on the day [you had the dream]. However, you are not required to fast, for (the Amora) Samuel said: "Dreams speak falsehood," (Maseches Berachos 55b). You are, however, required to repent and to devote the entire day to the study of Torah and in prayer. Concerning fasting on Shabbos for a worrisome dream, see Orach Chaim, chapter 288.

footnotes: ¹⁷ A pregnant woman, or a woman after child-birth should not be told to fast, but, rather, to give charity in lieu of fasting. (Ibid