Just as it is a mitzvah for the congregation to fast and pray about its troubles, similarly it is a mitzvah for every individual to fast and pray about his troubles, for example, a family member who is sick, or somebody who is lost, or someone unjustly imprisoned. He should pray to Hashem and request that He save him. This is one of the methods of repentance; a person should never say, God forbid, that the trouble came about by chance, as it is written, “If you act with Me by chance (attribute the trouble to chance) then I will treat you with anger ‘by chance.’” To the contrary, a person should realize that Hashem brought this trouble upon him on account of his sins. He should examine his deeds, and return to Hashem, and He will have mercy on him.
Even though this is the proper reaction to one’s own troubles, but one should not say this to others when they have troubles. The Shulchan Aruch states: “Just as one may not inflict monetary damages, one may not inflict with painful words. Inflicting with painful words is the worse of the two, because money can be returned, but words cannot be returned. Also, words affect one’s very self, whereas money affects one’s possessions only. Someone who cries out because of painful words is answered (by Hashem) immediately. What is an example of painful words? If his friend’s enduring affliction, he should not speak to him as Iyov’s (Job’s) friends spoke to him, saying ‘Remember — who has been innocent and destroyed?’ Nor should one say something similar to this.”
Initiating the fast: An individual who wants to fast must obligate himself at Minchah on the preceding day. One may eat and drink after this Minchah until dawn.
How does one obligate one’s self to fast? In the Amidah one inserts the following in Shomei’ah t’fillah or before O’seh shalom: I hereby accept upon myself to fast a private fast tomorrow. May it by Your will to accept my prayers on my fast day.” According to the R”ma it is preferable to say this before O’seh shalom. Some say a different version of acceptance that is printed in the siddur (prayer book).
One who fasts on a Monday, Thursday and succeeding Monday, can accept upon himself these fasts by answering Amen to the Mi Shebeirach that the chazzan says on proclaiming them on the preceding Shabbat, even though these fasts are not three days in a row. It is better, though, to explicitly accept these fasts at the preceding Minchah, since merely answering Amen is not considered a vow.
Someone who always fasts during the ten days of repentance, on the day preceding Rosh Chodesh, or the day preceding Rosh Hashanah, needn’t accept the fast upon himself explicitly, since he accepts them automatically because this is the custom of many Jews.
One who dreamed a bad dream should fast that very day, even though he didn’t accept the fast at Minchah the preceding day. A person shouldn’t fast on account of a dream without consulting with a rabbi.
Somebody who decided to fast in his mind only but did not say an acceptance should be stringent upon himself and fast as he decided, even though he didn’t decide this at Minchah on the preceding day. However, if he decided after sunset that he would fast in the morning, he needn’t be stringent.
If, at night, one accepted upon one’s self to fast the next day, then even though this is not considered acceptance insofar as the following paragraphs, nevertheless some say that one is obligated to fast the next day.
“Borrowing” a fast: Someone who vowed to fast one or more days, and didn’t specify which days, even if he said at Minchah later that he would fast the next day, if something unexpected comes up that requires him to eat, he can “borrow” his fast and eat (even if it came up the next day and he already started fasting), as long as he reschedules his fast for another day. Some examples of justified reasons for this are an invitation to a meal honoring a mitzvah (even if somebody else is doing the mitzvah), an invitation by a very honorable person to a meal (and circumstances make it difficult for him to refuse the invitation), or the fast is very difficult for him (for instance, an unusually hot day). Let us emphasize that this is possible solely because when he originally vowed that he would fast he did not specify which day he would fast. However, if one accepts at Minchah to fast the next day with no previous vow whatsoever, then one must fast as one accepted and doesn’t have the option of “borrowing” one’s fast and postponing it to a later date, even if the fast turns out to be quite difficult.
If one didn’t accept at Minchah a fast when he needed to, then even though one did fast the next day, it is not considered to be fulfilling one’s vow.
Redeeming one’s fast: Someone who accepted on himself to fast and finds that the fast is very difficult for him, can redeem the fast by donating charity to the poor, in accordance with his financial standing. However, someone who vowed that he would fast cannot redeem his fast.
How much money is required to redeem one’s fast? Some authorities say it’s the amount of money that he spends on his food that day. Other authorities say that only poor people donate that amount, but someone more affluent should donate more.
One can neither redeem nor “borrow” one of the five annual fasts, nor any other public fast that was proclaimed by the public leaders. However, the leaders can make an explicit caveat when they proclaim a public fast that those who find it difficult to fast may redeem it.
Fasting on a Monday-Thursday-Monday: Someone who vowed to fast on a Monday, Thursday, and the next Monday, but didn’t specify the exact days when he will fast, can exchange them with Thursday-Monday-Thursday, but may not switch to other days of the week. This is because he probably chose those days of the week because they are days of judgment.
Somebody who traditionally fasts on the Monday, Thursday, and succeeding Monday after Pesach (Passover) and after Sukkot, or usually fasts during the Ten Days of Repentance, or on the day before Rosh Chodesh, and didn’t mention this at the preceding Minchah because he relied on the fact that it is a recognized custom to fast on these days, may eat (and it is a mitzvah to eat) at a circumcision or Pidyon Haben (redemption of the firstborn son) or any other meal honoring a mitzvah. This is true even if he intended to fast when he answered Amen to the Mi shebeirach blessing those who fast those fasts, and he does not need to formally annul his vow. This is because every person who fasts these fasts and did not say so explicitly has in mind to fast them in accordance with the general custom, and when this custom was started an exemption was made for those who partake in a meal honoring a mitzvah.
After eating at a meal honoring a mitzvah, one may eat regularly the rest of the day as well, because the fast has been broken. However, before the meal, one may not eat. Only the father and mother of the baby boy and the sandek may eat before the meal on the day of a circumcision, since that day is considered their holiday.
Fast of a Yahrzeit: The first time one observes the fast on the yahrzeit (date of death) of one’s parents one should make a condition that if the yahrzeit will fall on a Friday then one will fast only until after praying Maariv. In such a year, one should pray Maariv early and eat early in order to not enter Shabbat while still fasting.
If one did not make this condition, and one year the yahrzeit did occur on Friday and one fasted until night, then this shows that he accepted upon himself to complete the fast even on Fridays. If, even after many years, the yahrzeit recurs on a Friday, then one is obligated to fast until night, and any condition made in the meantime has no validity.
If one did not make the aforementioned condition but the yahrzeit has not yet fallen on a Friday, some authorities hold that one must fast until night even on a Friday. Other authorities are of the opinion that since he hasn’t fasted yet on a Friday he can still make a condition.
Someone who usually fasts on every yahrzeit can fast even without accepting the fast upon himself explicitly.
Selected laws of private fasts: A person who is fasting shouldn’t pamper himself, nor joke, nor be happy or joyous, as it says, “What should a living person complain about? A person should complain about his own sins.”
It is permissible to wash one’s mouth with water on a private fast.
Privacy of fasting: Someone who is fasting and boasts about his fast will be punished. However, if people offer him food and press him to eat, he may reveal that he is fasting.
The Mishnah B’rurah wrote, “It is a mitzvah to publicize that he is fasting on a public fast, in order to reinforce the fast so that others will follow him, and not in order to boast, Heaven forbid. If someone must eat on a public fast because of illness, Heaven forbid, he shouldn’t publicize that he ate.
Aneinu on a private fast: Someone fasting a private fast (whether voluntarily or in order to nullify a bad dream) inserts Aneinu in Shomei’a t’fillah at Minchah, as on public fasts. Even though the Aneinu is written in the plural and his fast is individual, he should nonetheless say it as usual, in order not to change from the version that the sages authored.
If a person should have accepted the fast on the previous day but didn’t do so, according to the Shulchan Aruch he cannot say Aneinu in his prayer, even though he actually fasted. According to the R”ma he can say Aneinu.
Ate by accident: Someone who ate on a fast (when forbidden), whether by mistake or purpose, is obligated to complete the fast, even though he ate. He should ask a rabbi as to how many fasts he should undertake in order to atone for his eating on a fast day. If had vowed to fast that day, then all the more so must he complete the fast.
If one ate an olive’s size of food or drank a r’vi’it of liquid, one doesn’t say Aneinu in his prayers on a private fast.
Trouble has passed: An individual who accepted upon himself to fast because of a trouble and the trouble has passed, or to fast for the recovery of a sick person and the person recovered or passed away (God forbid), must observe all of the fasts that he accepted upon himself. Similarly, if he accepted upon himself fasts or a certain mitzvah until his son becomes bar-mitzvah (thirteen years old) and the son passed away beforehand, he must fulfill his words until the time when his son would have become bar-mitzvah.
If one is informed that the reason for one’s fast had already passed before one accepted upon oneself to fast, this is considered “a mistaken acceptance,” and one need not fast.
If one accepted upon oneself to fast on condition, for instance, “if this patient recovers then I will fast ten days,” and the patient passed away (God forbid), then one needn’t fast.
Fast to annul a bad dream: A fast together with repentance is an excellent combination to annul a bad dream, like fire to a piece of straw — but it has to be on that same day as the dream. Nevertheless, there is no obligation to fast, as Shmu’el said, “The predictions of dreams are nonsense.” Still, one is obligated to repent and busy oneself that day with Torah and prayer. We fast to annul bad dreams on certain special dreams only, and it is proper that a person not fast a lot because of dreams: rather one should pray to Hashem that He should have mercy on him. If one does want to fast then one should consult with a rabbi, especially if the dream was on Shabbat.
Fasting several hours: A person who did not accept upon himself to fast, and in the middle of the day noticed that he hadn’t eaten anything yet and would like to remain fasting until night, and accepted upon himself to fast the rest of the day, is called keeping a “fast of hours.”
According to Maimonides, at such a “fast of hours” one says Aneinu, since he did accept upon himself this short fast before he started it. However, if he ate or drank in the morning and then decided to fast for the remainder of the day, then although this, too, is considered a “fast of hours,” he does not say Aneinu.
Someone who accepted upon himself to fast and didn’t mention how many hours he would fast must fast the entire day until night (tzeit hakochavim, when the stars are visible), even on a Friday.
Someone who accepted upon himself a private fast, and made a condition at the time of acceptance that he will fast only until praying Maariv, may eat after praying Maariv, even if he prayed Maariv early (after P’lag Haminchah), and concluded before sunset. According to the R”ma, he may eat after praying Maariv even without making a special condition — still, it’s better to make a condition.