Comforting a mourner on Shabbos:[1]

It is permitted to comfort an Avel on Shabbos.[2] [However, one is not to publicly announce for people to go be Menachem Avel, due to the prohibition against showing public Aveilus on Shabbos.[3]]

The Nussach:[4] It is even permitted to comfort the Avel in the same wording as used during the week “May Hashem comfort you.”[5]

The law Lechatchila: It was with difficulty that the Sages allowed comforting an Avel on Shabbos.[6] It is therefore improper to do like those who do not visit the Avel throughout the week, and only visit him on Shabbos.[7]

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[1] See Admur 287:2-3; Shabbos 12a; Kaf Hachaim 287:1-4; Piskeiy Teshuvos 287:1; Pnei Baruch 11:10

[2] Admur 287:2; Michaber 287:1; Tur 287:1; Beis Hillel in Shabbos 12a

Other Talmudic opinions: According to Beis Shamaiy, it is forbidden to comfort mourners on Shabbos. [Shabbos ibid] This is due to the pain that it arouses amongst the comforters. [Rashi Shabbos ibid]

[3] P”M 287 M”Z 1; M”B 287:3

[4] Admur ibid; M”A 287:1 in name of Tur; Tur Y.D. 393:3 in name of his father the Rosh that so is the custom in Ashkenaz to perform an entire comforting ceremony for the Avel on Shabbos [see there for the full details] and they would tell him Hamakom Yinachamecha; 2^nd opinion in M”B 287:1; Aruch Hashulchan 287:3; Kaf Hachaim 287:4

Other opinions: Some Poskim rule one is to change the normal form of comforting on Shabbos and say, “Shabbos Hi Melinachem Unechama Kerova Lavo.” [Bach 287 in name of Rashal in his Hagahos Hatur, brought in M”A 287:1; Taz 287:1; P”M 287 M”Z 1; 1^st opinion in M”B 287:3; Kaf Hachaim 287:4 in name of Rashal and Bach; See Michaber and Tur 287:1 who writes that one is to change for the normal way said during the week, however, the Nussach they then bring refers only to Bikur Cholim and not Nichum Aveilim-See M”B ibid and Kaf Hachaim 287:4] Other Poskim write that the custom is for one to simply say “Shabbat Shalom” and not more. [Kneses Hagedola 287:2; Kaf Hachaim 287:4] Others say the custom is not to say anything. [Mateh Yehuda 287:1; Kaf Hachaim 287:4]

[5] The reason: As through saying it in this language, which is a request that Hashem comfort him, one will not come to arouse him to cry. [Admur ibid; Perisha Y.D. ibid; Machatzis Hashekel 287:1]

[6] Admur 287:3; M”A 287:1; Shabbos 12b; Elya Raba 287:1; Machazik Bracha 287:3; M”B 287:1; Kaf Hachaim 287:1

The reason: As this may lead one to become saddened together with the Avel. [Admur ibid; M”A ibid]

[7] Admur ibid; M”A 287:1; M”B 287:1

[8] Aruch Hashulchan 287:3 “By us it is not customary to perform Nichum Aveilim on Shabbos”; Nimukei Orach Chaim 287:1; Daas Kedoshim 395:1; Darkei Hachaim 4:13; Gesher Hachaim; Kol Bo [Greenwald]; Pnei Baruch 11 footnote 21; Piskeiy Teshuvos ibid; Nitei Gavriel 86:8

[9] Piskeiy Teshuvos ibid based on Nimukei Orach Chaim ibid who writes that back then it was mainly done to lift the spirits of the Avel; Seder Hayom; See Shevet Halevi 4:53 who questions the assertion of Nimukei Orach Chaim and states that perhaps this custom to not do so on Shabbos is not correct as none of the Poskim record it, and all the arguments that he brings against it were around in Talmudic times as well, and nevertheless it was permitted ; See Betzeil Hachochma 2:44; See Orchos Rabbeinu 4:118 that the Steipler was Menachem Avel on Shabbos with the changed Nussach