Omissions from the prayer on Erev Yom Kippur:
The following prayers are omitted on Erev Yom Kippur:
- Mizmor Lesodah
- Avinu Malkeinu.
- Diminish in Selichos
Is Tachanun said by Kerias Shema Sheal Hamitah?
Those who do not decrease in the amount of Selichos recited on the night of Erev Yom Kippur are to say Tachanun at night in Kerias Shema Sheal Hamitah. However, those who are accustomed to decrease in the amount of Selichos said on Erev Yom Kippur, are not to say Tachanun at night. Regarding the Chabad custom: Some write one is not to say Tachanun by Kerias Shema Sheal Hamitah, however this matter requires further analysis.
Does one hit his chest in Shemoneh Esrei of Shacharis or Mincha on Erev Yom Kippur?
Some write one is not to hit his chest during Shemoneh Esrei of Erev Yom Kippur. Others however say they witnessed the Rebbe doing so.
C. Mizmor Lesodah:
The Psalm of Mizmor Lesodah which is normally recited after Baruch Sheamar, is omitted on Erev Yom Kippur.
D. Avinu Malkeinu:
Avinu Malkeinu is omitted both in Shacharis and Mincha of Erev Yom Kippur.
Erev Yom Kippur that falls on Friday: Some communities have the custom to say Avinu Malkeinu by Shacharis of Erev Yom Kippur in years that Yom Kippur falls out on Shabbos, in which case Avinu Malkeinu is not recited on Yom Kippur. [Practically, the Chabad custom is to omit Avinu Malkeinu even when Erev Yom Kippur falls on Friday.]
E. Diminish in Selichos:
Those who recite Selichos during Aseres Yimei Teshuvah, diminish in the amount of Selichos recited on Erev Yom Kippur before Alos Hashachar. Some places however are accustomed to increase in Selichos prior to Alos being they do not consider it a Yom Tov at night and rather only from Alos and onwards. Practically each place is to follow its custom. [The Chabad custom is not to recite Selichos at all during Aseres Yimei Teshuvah as explained in Chapter 1 Halacha 5B.]
 604/4; Siddur Admur
 The reason: Although from the letter of the law Erev Yom Kippur is not a Yom Tov at all [and hence Tachanun should be recited], nevertheless the custom is to treat it like a Yom Tov. [Admur ibid]
 Siddur Admur
 Admur 604/5; Siddur Admur; M”A 604/1; Levush 131/6
 The reason: As the Yom Tov of Erev Yom Kippur is only accustomed to begin from nightfall even according to those opinions who view the night of Yom Kippur as a Yom Tov and thus decrease in Selichos and eat meat on the night of Erev Yom Kippur. [604/5]
 Based on Admur 604/3
 Hiskashrus footnote 15
 Tzaruch Iyun on the above ruling in Hiskashrus as the Chabad custom is to never say Selichos during Aseres Yimeiy Teshuvah, hence what proof is there that we consider it a Yom Tov from the night as hold some opinions? To note that in the Siddur Admur writes that Kaparos is followed after Selichos, hence showing that Selichos is said at the night of Erev Yom Kippur. However, it is unclear if it is a normal amount of Selichos or a lesser amount.
 See Hiskashrus 948 footnote 48
 Admur 604/6; Siddur Admur regarding all days that Tachanun is omitted; Rama 604/2
 The reason: As in this Psalm we mention “Liyom Tzarah” and Erev Yom Kippur is not a day of pain. [604/6; Levush 604/2]
 Takana of Rebbe Rayatz, recorded in Siddur Tehillas Hashem prior to Musaf Davening of Shabbos.
 Admur 604/6; 1/17; 51/1; Siddur Admur; Rama 604/2
Other Opinions: There are opinions who hold that Mizmor Lesoda is recited on Erev Yom Kippur. [Kaf Hachaim 51/52]
 The reason for the omission: This is due to that the Toda sacrifice was not allowed to be brought on Erev Yom Kippur being that it diminishes its eating time. [Kama 1/17; 51/1] To note from Chidushei Tzemach Tzedek p. 208 who differentiates between Erev Yom Kippur and Erev Pesach, explaining the Toda offering was allowed to be brought on Erev Yom Kippur and hence Mizmor Lesoda should be recited. This concept of that the Toda was offered on Erev Yom Kippur is also accepted and defended by the Rebbe in Igros Kodesh 3 p. 71-72. Vetzaruch Iyun as why no mention was made of Admur’s clear ruling that the Todah was not brought.
 Admur 604/7; Rama 604/2
 Custom of Krakow. [Rama ibid]
 See Admur 619/11
 Shevach Umoadim 22/1 from Sefer Haminhagim [I have not found the source for this is Sefer Haminhagim]; Hiskashrus based on Luach Kolel Chabad. [To note that in Otzer Minhagei Chabad they printed a typing error saying that the Chabad custom is to say Avinu Malkeinu when in truth it is our custom not to say it.]
 Admur 604/4; Rama 604/2