Shabbos and Yom Tov:[1]

It is customary [amongst Ashkenazi Jewry] for an Avel^[2] [of a father or mother or of other relatives[3]] not to lead the prayers as Chazan on Shabbos or Yom Tov [throughout the Shiva and the mourning period of Shloshim or 12 months[4]], even though there is no prohibition in the matter. [Due to the custom, it is forbidden to do so.[5] One is not to lead the prayers for even Pesukei Dezimra.[6] If there is no better Chazan available, then an Avel may lead the prayers even within Shloshim.[7] Sephardim, however, do not follow the above custom, and hence lead the prayers even on Shabbos and Yom Tov.[8]]

Mincha Erev Shabbos:[9] The Avel is to lead the prayers also for Mincha of Erev Shabbos, even if it is being prayed very close to Shabbos.

Kabalas Shabbos: It is customary for the Avel not to lead the prayers even for Kabalas Shabbos [prior top Maariv].

What is the law during the 13^th month of a leap year?[10] If the year one’s parents passed away is a leap year, then after 12 months the son may Daven for the Amud on Shabbos/Yom Tov even though it is still prior to the first Yahrzeit.

Yahrzeit: If one has a Yahrzeit of one parent on Shabbos or Yom Tov during the 12 months of Aveilus for another parent [or during Shloshim of other relative] he is nevertheless to Daven for the Amud as usual.[11] When the Yahrzeit falls on Erev Shabbos, or Shabbos, one says the Kaddish Yasom that follows the psalm of Mizmor Shir Leyom Hashabbos which concludes Kabalas Shabbos.[12] [Many are accustomed to also Daven for the Amud for Kabalas Shabbos.]

[1] Rama 376:4; Teshuvas Maharil

Other opinions: Some Poskim rule an Avel may Daven for the Amud on Shabbos and Yom Tov. [See Meir Nesivim 80, brought in Pischeiy Teshuvah 376:8]

[2] This refers to a person that is within 12 months of mourning for his father or mother [M”A 581:4], and a person that is within 30 days of mourning for other close relatives. [M”B 581:7; Kaf Hachaim 581:33]

[3] See previous footnote

[4] 12 months of mourning for his father or mother. [M”A 581:4] 30 days of mourning for other close relatives. [M”B 581:7; Kaf Hachaim 581:33]

[5] Shach 376:14

[6] See Nitei Gavriel 56:11

[7] Shach 376:14; Taz 376:2; Maryu 47

[8] Yalkut Yosef Aveilus p. 533

[9] See Poskim in Nitei Gavriel 56:10

[10] P”M 660 M”Z 2; Piskeiy Teshuvos 581 footnote 81; Nitei Gavriel 56:8

[11] Igros Kodesh 14:430 [printed in Shulchan Menachem 5:304]; Minchas Yitzchak 9:134; Betzeil Hachochmah 4:199; See Hiskashrus 569:18; Dudaei Hasadeh 44; Vayilaket Yosef 14:41; Poskim in Nitei Gavriel 56:2; Piskeiy Teshuvos 287 footnote 73; However, see Divrei Malkiel 6:23

[12] Sefer Hamamarim 5708 p. 147; Sefer Haminhagim [English] p. 52 and 180 regarding Yahrzeit falling on Friday and footnote 213 regarding Shabbos; Igros Kodesh 3:20 regarding both Shabbos and Friday [brought in Shulchan Menachem 5:327]

The reason: If the Yahrzeit falls on Friday he is to say the Kaddish as ideally Melacha is permitted until Barchu is recited, and it is hence part of the Friday prayers. [footnote of Rebbe in Sefer Haminhagim ibid based on Admur 261:7] It goes without saying that this Kaddish is recited by one whose Yahrzeit is on Shabbos as it is said as part of the Shabbos prayers, as well as that it is today always said after sunset when the Yahrzeit has already begun. [See Shulchan Menachem ibid footnote 10]