Must pregnant or nursing women fast? Pregnant and nursing women are exempt from fasting. Nevertheless the custom is for even pregnant and nursing women to fast unless they are in great pain [or feel weak] in which case they are specifically not to fast. They are likewise not required to make up the fast at a later date. [Practically the widespread custom today is for pregnant women not to fast as they are all considered weak. They are also not required to make up the fast at a later date.]
Must one fast if he is sick or weak? One who is sick is exempt from fasting. One who is old or weak is not required to fast if he receives a medical order from a doctor that the fast is detrimental to his health. He is not required to make up the fast on a later date when he feels healthy.
Women after birth: According to all opinions a woman within 30 days of giving birth does not need to fast.
One who is traveling: If one feels healthy he must fast even if he is traveling that day.
Children: All children that are above the age of Bar or Bas Mitzvah are obligated to fast. All children below the age of Bar or Bas Mitzvah are not required to fast any of the four Rabbinical fasts. One is not even required to educate the child to fast for a certain amount of hours into the day. Some Poskim however rule that a child which has reached the age of understanding the mourning is only to be allowed to eat bread and water or other simple staple food. The custom however is not like this opinion. [However they are not to be given sweets and the like although one is not required to stop them from eating it if they are in the midst of doing so.]
From what stage in pregnancy is a pregnant woman exempt from fasting?
A pregnant woman that is in pain is exempt from fasting starting from the 41st day of her pregnancy. If she feels weak or in great pain then she may be lenient even prior to 40 days. If she does not feel weak or any pain at all then she is required to fast if she has not yet entered her second trimester.
Must a woman fast if she is after a miscarriage?
Must an old or weak person fast?
An old person or a person who is weak is not to fast on Taanis Esther or on any of the three Rabbinical fasts. Practically however one may not be lenient unless he receives a medical order from a doctor that the fast is detrimental to him.
Must a Chasan and Kallah within the 7 days of Sheva Brachos fast?
Taanis Nidcheh: When a fast day falls on Shabbos and was hence pushed off to Sunday a Chasan is required to fast the entire day as usual even if he is within the seven days of Sheva Brachos. Others however rule he is not required to fast until nightfall.
May one who is not fasting eat like a normal day?
It is proper to diminish somewhat in eating and drinking if one is not fasting that day, such as a pregnant or nursing woman, in order to participate in the day of oppression followed by the community.
Must a soldier in the army fast?
If he is not involved that day in any military operation or warfare then he must fast like everyone else. If however he is taking part in a military operation then he must eat in order to have strength.
If fasting on Tzom Gedalia may cause one to be unable to fast on Yom Kippur is he nevertheless to fast?
If one forgot and ate or drank on the fast day is he to continue fasting?
 Rama 550/1
 See Q&A
 Some Poskim rule that all women within 24 months after birth are defined as nursing women, even if they are not actually nursing anymore. [Daas Torah 550; Piskeiy Teshuvos 550/1]
 M”B 550/5
 See Piskeiy Teshuvos 550/1; Piskeiy Teshuvos 686/3 that so is the ruling of today’s Rabbanim. So rules Ashel Avraham Butchach 686 Mahadurah Tinyana; Hisorerus Teshuvah 1/6; Divreiy Yatziv 2/291
 M”B 686/5; Kaf Hachaim 686/22 regarding Taanis Esther and the same would apply regarding the four fasts; See however M”B ibid in name of the Bach.
 Even if there is no danger involved in his illness. [Chayeh Adam 133/6; M”B 550/4]
 Chayeh Adam 133/6; M”B 550/4; Kaf Hachaim 550/7
 Ruach Chaim 550/1; Kaf Hachaim 550/6; However without a direct medical directive he is not to be lenient. [ibid]
 See Rama 586/2; M”B 686/5; Shaareiy Tziyon 586/11; Kaf Hachaim 686/22
 See M”B 686/4; Kaf Hachaim 686/19
 M”A 668/4 brought in M”B 686/6
 Admur 616/9; Elya Raba 549/7; Kaf Hachaim 550/1
Two hairs: The above law only applies if the child has reached puberty which means that he or she has grown two pubic hairs. If the child has reached this age but does not have two pubic hairs, then although he or she must keep all the commands out of doubt that perhaps he or she grew the hairs and they fell off, nevertheless the child is not required to fast any of the Rabbinical fasts. [Admur ibid]
Other Poskim: Some Poskim rule the child must fast even the Rabbinical fasts once he or she reaches 13/12 even if the child has not yet grown two hairs. [P”M 550 A”A 2; Biur Halacha 550 “Hakol”; Kaf Hachaim ibid]
 Admur 616/9; Elya Raba 549/7; M”B 550/5; Kaf Hachaim 550/1
 Chayeh Adam 133/6; M”B ibid; Kaf Hachaim 550/9; Piskeiy Teshuvos 550 footnote 10
Other Opinions: Some Poskim rule that children [of nine years old] are to be educated to fast for a few hours into the day just as is the law on Yom Kippur as rules Admur 616/5. [Erech Hashulchan 554/2 and Beis Hillel brought in Kaf Hachaim 554/23; Chanoch Lenaar 21 footnote 9; Beir Moshe 8/98; Piskeiy Teshuvos 554/9]
 M”A 550/2; Rameh 111; P”M 549 A”A 7; Chayeh Adam ibid; M”B ibid; brought in Kaf Hachaim 549/8 and 550/9
 Some write this is starting from age nine. [See Piskeiy Teshuvos 550 footnote 11]
 Birkeiy Yosef 549/1; Daas Torah; Kaf Hachaim 549/8 and 550/9; Beir Moshe 8/95
 Beir Moshe ibid; Piskeiy Teshuvos 550/3
 M”B 550/3; Kaf Hachaim 550/5; Mor Uketzia 550; Piskeiy Teshuvos 550/1; Nitei Gavriel Bein Hametzarim 5
The M”B ibid rules that seemingly she is considered pregnant starting from the 4th month and onwards, however possibly she is considered pregnant even starting from the 41st day if she feels pain. However less than 40 days she is considered like any other women that must fast unless she is in great pain. He then concludes that if she feels weakness she is not to be stringent at all. The Kaf Hachaim ibid however writes this ruling a little differently: He writes that if she is in great pain she may be lenient from the 41st day, however prior to 40 days she must fast even if she is in great pain.
 See Background!
 Piskeiy Teshuvos 550/1; There he brings that they are exempt from fasting for a full 24 months after the miscarriage just like a woman who gave birth.
 Kaf Hachaim 550/6 in name of Ruach Chaim 550/1
 As they are no different than a pregnant or nursing women which is exempt from fasting. [Kaf Hachaim ibid]
 Ritva Taanis; Peri Hadamah 4/41; M”A 550/5; Kesones Yosef 5; Beis Yehuda 31; Birkeiy Yosef 549/2; Kaf Hachaim 549/9
The reason: As the mourning of the public differs the Holiday of an individual. [Ritva ibid]
Other Poskim: Some Poskim are lenient in this matter and rule a Chasan is not required to fast, as his Yom Tov is greater than the Yom Tov of Mila and hence pushes off Aveilus. [Beis David 476 brought in Kaf Hachaim ibid]
 M”A 559/11 in name of Kneses Hagedola; brought in Shaareiy Tziyon 559/34
 Shaareiy Tziyon 559/34 in name of Beis Yehuda brought in Pischeiy Teshuvah
 Ashel Avraham 686 Mahadurah Tinyana
 See Sdei Chemed Mareches Yom Kippur 1/10 for an full analysis on this subject; Viheishiv Moshe 16 writes that this is dependent on the dispute between the Ridbaz and Chacham Tzvi brought in Beir Heiytiv 90/11 regarding a prisoner who has ability to Daven today with a Minyan or to choose to Daven on R”H with a Minyan.
 Lehoros Nasan 8/34
 Halachos Ketanos 1/290; Ohel Moshe ibid; Beir Moshe 8/34 that the world is accustomed to be lenient; Piskeiy Teshuvos 603/1
 Beir Moshe 8/34
 Kaf Hachaim 549/6-7