Blessings, Kiddush and Birchas Hamazon for one who eats on Yom Kippur:

Kiddush and Lechem Mishneh:[1] One who is [lethally] ill [or contains a medical condition, of a caliber that he is] required to eat on Yom Kippur [such as a woman after birth[2]], does not need to say Kiddush over wine or bread [prior to eating], and is not required to perform Hamotzi over two loaves of bread [i.e. Lechem Mishneh].[3] [This applies even if Yom Kippur falls on Shabbos.[4] This likewise applies to a child below Bar/Bas Mitzvah who is not fasting.]

Blessings:[5] One who is required to eat on Yom Kippur due to medical reasons [or if he is a child below Bar/Bas Mitzvah] is required to say a blessing before and after eating the foods [if he ate a Kezayis within four minutes, and has the physical and mental capability of doing so[6]].[7] [One who is eating according to Shiurim of less than 30cc and less than 30 grams every nine minutes, is not to say an after blessing at all.]

Yaleh Veyavo in Birchas Hamazon:[8] A sick person [who is required to eat on Yom Kippur due to medical reasons, or a Yoledes, or a child below Bar/Bas Mitzvah[9]] who eats a Kezayis of bread [within four minutes] on Yom Kippur, is to recite Birchas Hamazon if he has the physical and mental capability of doing so. In Birchas Hamazon, he/she is to recite Yaleh Veyavo prior to Uvinei Yerushalayim and say “Beyom Hakippurim Hazeh.” If Yom Kippur falls on Shabbos, then one is to also recite Ritzei.[10] [However, some Poskim[11] rule that based on Admur in the Siddur one is not to recite Yaleh Veyavo in Birchas Hamazon. Practically, it is to be recited.[12]]

Forgot to recite Yaaleh Veyavo/Ritzei:[13] If one forgot to recite Yaaleh Veyavo [or Ritzei] in Birchas Hamazon on Yom Kippur [or Yom Kippur that falls on Shabbos] and remembered only after beginning the blessing of Hatov Vehameitiv, he is not to go back and repeat Birchas Hamazon.[14] If, however, he remembered prior to beginning the blessing of Hatov Vehameitiv, after concluding Boneh Yerushalayim, then it is disputed[15] if he should say a special blessing on behalf of Yom Kippur [and Shabbos] and practically, no blessing is to be said and he is to continue with Hatov Vihameitiv.[16]

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[1] Admur 618:18; M”A 618:10; Shaar Hatziyon 618:22; Alef Hamagen 618:18; M”B 618:29; Har Tzevi 1:155; Piskeiy Teshuvos 618:15

Other opinions regarding Lechem Mishneh: Some Poskim rule Hamotzi is to be recited over Lechem Mishneh on Yom Kippur. [Kneses Hagedola 618, brought in M”A ibid; Mor Uketzia 618; Kaf Hachaim 618:60; Meishiv Davar 1:359; Piskeiy Teshuvos 618:15] The above Poskim negate this matter.

[2] M”B 618:28

[3] The reason: As the Sages did not establish these matters [of Kiddush and Lechem Mishneh] on Yom Kippur. [Admur ibid; M”A ibid]

[4] Implication of Admur ibid [As this ruling of Admur is in continuation of the law of Yom Kippur falling on Shabbos and reciting Ritzei, and he then says on this case that Kiddush and Lechem Mishneh is not required]; Poskim brought in Shaar Hatziyon 618:22; Alef Hamagen 618:18; Har Tzevi 1:155; Piskeiy Teshuvos 618:15

Other opinions regarding Kiddush on Shabbos: Some Poskim rule Kiddush is to be recited when Yom Kippur coincides with Shabbos. [Rav Akiva Eiger brought in Shaar Hatziyon 618:22] The above Poskim negate this matter.

[5] Admur 618:18 regarding Birchas Hamazon; 196:5 regarding an Issur food and that this applies according to all; 204:15 regarding Yom Kippur and Issur food; Seder 7:9 and Luach 10:9 regarding Yom Kippur and Issur food; Michaber 196:4 and 204:9 regarding Issur; Tur 618 regarding Yom Kippur; M”A 204:21; M”B 604:46; All the following Rishonim regarding Yom Kippur: Maharam Merothenberg 71; Tashbeitz 565; Hagahos Maimanis Brachos 2:2; Rosh, brought in Beis Yosef 618 and Taz 618:1 regarding Birchas Hamazon

Other opinions: Some Poskim rule that one does not recite a before or after blessing upon eating an Issur even in a case of fatal illness. [Dispute recorded in Taz 618:10; Panim Meiros 2:6, brought in Shaareiy Teshuvah 196:2]

[6] Admur 618:18

[7] The reason: One who eats a forbidden food in a case of danger is required to say a before and after blessing over the food as he in truth is eating permitted [food], as he is doing a Mitzvah to save his life. [Admur 196:5; See Piskei Dinim Tzemach Tzedek 7:1; Likkutei Sichos 3:985; 19:35] The same applies for a sick person who eats on Yom Kippur. A blessing is needed before and after as he has nevertheless benefited from the food despite the fact that he has benefited against his will, and under the duress of his illness. [Admur 204:15]

[8] Admur 618:18; 1st opinion and final ruling in 188:11; Michaber 618:10; Maharam Mirothenberg 71; Tur 618 in name of Rosh; Hagahos Maimanis; Rashba; Chayeh Adam; Mateh Efraim; Piskeiy Hasiddur 185; 1^st opinion in M”B 618:29

Other opinions: Some Poskim rule that a sick person who ate on Yom Kippur is not to recite Yaaleh Veayavo/Yom Kippur in Birchas Hamazon. [2^nd opinion in Admur 188:11; Taz 618:10; M”A 188:10 in name of Shivlei Haleket 312 in name of Rav Avigdor Katz; Shaar Hakolel 34:9 based on omission of Admur in Siddur and other Poskim such as Tanya Rabasi and Elya Raba; 2^nd opinion in M”B 618:29]

[9] Mateh Efraim 618; M”B 618:

[10] Admur ibid; M”A 618:11; Kneses Hagedola 618

[11] Shaar Hakolel 34:9 based on the fact that Admur in the Siddur omitted Yaaleh Veyavo of Yom Kippur

[12] Piskeiy Hasiddur 185 [see there in length for negation of ruling of Shaar Hakolel ibid]; See however Hiskashrus who writes it is to be recited in Harachaman, See Nitei Gavriel Bein Hametzarim 66:11

[13] Admur 188:11

[14] Admur ibid; M”A 188:7; Elya Raba 188:8; Yad Aaron on Tur 188; Kitzur SHU”A 133:18; M”B 618:29

The reason: As Birchas Hamazon is not an obligation for him due to the holiness of the day, and is simply done due to his illness. [Admur ibid; Poskim ibid]

[15] Some Poskim rule that he is to say the blessing of “Baruch Ata Hashem Elokeinu Melech Haolam Asher Nasan Yamim Kedoshim Leamo Yisrael Es Yom Hakkipurim Hazeh.” However, he is not to say a concluding blessing. [1^st opinion in Admur ibid; M”A 188:7; Elya Raba 188:8; Yad Aaron on Tur 188] Other Poskim rule he is even to say a concluding blessing of Mikadesh Yisrael Veyom Hakippurim. [2^nd opinion in Admur ibid; Maharil 146; Kneses Hagedola 188; Elya Zuta 188:7; Nehar Shalom 188:4] Other Poskim rule that he is not to say any blessing at all, as even initially Yaaleh Veyavo is not to be recited. [3^rd opinion in Admur ibid; All Poskim in previous Halacha who rule Yaaleh Veyavo is not said] Practically, one is to suspect for the last opinion in order not to enter into a Safek Bracha Levatala, and therefore he should not say the blessing of Asher Nasan at all if he forgot to recite Yaaleh Veyavo. [Admur ibid]

[16] Conclusion of Admur ibid as explained in previous footnote; Implication of M”A 618:10; P”M 618 A”A 10;

[17] Levushei Mordechai Tinyana Y.D. 11; Yad Sofer 7; Shevet Halevi 8:139; SSH”K 39:31; Lehoros Nasan 2:42; Shraga Hameir 5:45; Piskeiy Teshuvos 613:2; 618:15

[18] Siddur Yaavetz

[19] Shaar Hakolel 34:9