Shehechiyanu by Candle lighting and Kiddush:
We do not recite Shehechiyanu during Kiddush [or candle lighting] of Shevi’i or Acharon Shel Pesach.
Order of Davening when Pesach falls on Friday evening:
Mincha: Prior to Mincha, Hodu is omitted although Patach Eliyahu is recited.
Maariv: When Yom Tov falls on Friday evening, one begins the Maariv prayer from Mizmor Ledavid [psalm 29], [omitting all the Psalms from Lechu Neranina until Mizmor Ledavid]. [One recites the entire prayer from Mizmor Ledavid and onwards, including Ana Bekoach; all the stanzas of Lecha Dodi; Mizmor Shir, Kegavna. In Lecha Dodi, the wording of Besimcha instead of Berina is recited. In Shemoneh Esrei one needs to mention “Shabbos” in the beginning of the middle blessing, and in the conclusion of the blessing. After Shemoneh Esrei one recites Vayechulu. One then recites Meiyn Sheva. After Mieyn Sheva one recites Kaddish Tiskabel, Mizmor Ledavid. One then recites half Kaddish, Barchu and Aleinu.]
Kiddush when Pesach falls on Friday evening:
Shalom Aleichem, Eishes Chayil, Mizmor Ledavid, Askinu: When Yom Tov falls on Friday evening, the Chabad custom is to say the following passages prior to Kiddush in an undertone: Shalom Aleichim, Eishes Chayil, Mizmor Ledavid Hashem Ro’i, Da Hi Se’udasa, Askinu. [The classical Chabad Hagados do not contain the above paragraphs prior to Kiddush, and they rather begin straight with Askinu and Vayechulu. Practically, the above paragraphs are to be recited from a regular Siddur, and one may then continue with the Kiddush written in the Haggadah.]
Extra words of Shabbos: When Yom Tov falls on Friday evening, one is required to add the extra words that mention Shabbos, in the paragraph of Kiddush, and conclude the blessing of Kiddush with Mikadesh Hashabbos ViYisrael Vihazmanikm.
Forgot to mention Shabbos in Kiddush: If one forgot to add any of the parts for Shabbos in the Kiddush, then he does not fulfill his obligation, and must repeat the Kiddush. Likewise, If one accidentally said the Friday night Shabbos Kiddush and not the Kiddush for Yom Tov, he does not fulfill his obligation and must repeat the correct Kiddush. [If he already drank the cup of wine, then he is to pour a new cup and recite Kiddush again.] However, if he concluded the blessing with Mikadesh Hashabbos Viyisrael Vihazmanim, then even if he did not mention Shabbos in the main paragraph the Yom Tov Kiddush, he nevertheless fulfills his obligation. Furthermore, if he mentioned Shabbos anywhere in the main paragraph, then even if he did not conclude the blessing with Mikadesh Hashabbos Viyisrael Vihazmanim, he nevertheless fulfills his obligation according to some Poskim, and is hence not to repeat it. This applies whether he concluded with only Mikadesh Hashabbos, or only Mikdesh Yisrael Viha.
 Admur 490:12; Michaber 49:7; Sukkah 47a
Discussing Shehechiyanu: The Rebbeim made a point of discussing the idea of omitting Shehechiyanu on Shevi’i Shel Pesach, hence implying a connection, at least through Torah, of Shehechiyanu to the day. [Otzer p. 218 based on Sichas 1983 Acharon Shel Pesach]
 The reason: As Shehechiyanu was already recited on the first two nights of Pesach. [Admur ibid; See Likkutei Sichos 37:14]
 Siddur Admur regarding Yom Tov; Ketzos Hashulchan 77:2; Shaar Hakolel 17:6 states that this was mistakenly omitted from certain prints of the Siddur
Other customs: Some are accustomed to beginning Maariv from after Lecha Dodi, by Mizmor Shir. [M”E 625:41]
 Siddur Admur
 The reason: Some write the reason is because there is a Mitzvah of Simcha on Yom Tov and we hence desire to speed the conclusion of Maariv. [Otzer Minhagei Yeshurun p. 64; See Admur 270:1 regarding Bameh Madlikin that it is omitted on Yom Tov in order to hasten Simchas Yom Tov] Alternatively the reason is because these Psalms contain the words Rina, and on Yom Tov we emphasize the words Simcha. [Ketzos Hashulchan 77 footnote 13] Alternatively, the reason is because the first five Mizmorim until Mizmor Ledavid relate to the five weekdays until Erev Shabbos while the psalm of Mizmor Ledavid relates to Erev Shabbos. Hence, we omit the first five Zemiros as it is not proper to relate them to Yom Tov. [Sichas Kodesh 2 p. 121]
 Shaar Hakolel 17:6; Ketzos Hashulchan 77 footnote 13; Otzer Minhagei Chabad p. 63
Other customs: Some are accustomed to only recite the first and last stanza of Lecha Dodi. [M”E 625:41] Others recite the first two stanzas of Lecha Dodi corresponding to Zachar and Shamor. [Alef Hamagen 625:56] Others recite the entire Lecha Dodi with exception to the stanza of Hisnaari Meiafar Kumi which is omitted. [Peri Megadim] Some are accustomed to omit Kegavna being that it mentions that all the other days are filled with wrath which is untrue regarding Yom Tov. [Siddur Yaavetz; Likkutei Mahrich Pesach; Alef Lamateh 625:67; Divrei Torah 9:72; Piskeiy Teshuvos 487:3]
 Hagahos Hasiddur of Rebbe Rashab; Ketzos Hashulchan 77:2; Mishnes Chassidim “Leil Yom Tov” 1:2].
Difference between Rina and Simcha: The term Rina denotes a bittersweet joy, a joy that comes as a result of a previous distance. However Simcha does not have any bitterness mixed with it at all. [Magen Avos Vayishlach’ Ketzos Hashulchan 77 footnote 13]
 If one forgot to mention Shabbos in any area, he must repeat the prayer. If it was mentioned anywhere in the prayer, it is valid, whether it was mentioned in the middle blessing or in the concluding blessing. [Admur 487:3]
 The above order is written in Piskeiy Hasiddur footnote 40; Otzer Minhagei Chabad p. 69
 Hayom Yom 19th Nissan, p. 46 [in current print]; Sefer Haminhagim p. 118 [English]; See Otzer Minhagei Chabad Nissan p. 142; Mateh Efraim 583:1; Likkutei Maharich 3:40; Kaf Hachaim 583:3; Piskeiy Teshuvos 473:2; Custom of Vizhnitz, Belz, brought in Piskeiy Teshuvos 473 footnote 9
Original Chabad custom: In the earlier prints  of Hayom Yom p. 46, it stated that one does not say Shalom Aleichem or Eishes Chayil at all on Shabbos Yom Tov, or Shabbos Chol Hamoed. This was based on an explicit directive of the Rebbe Rayatz that one is not to say it and that so was the custom of the Rebbe Rashab and the Rebbe Maharash, although there were some years in which they said it, but this is not relevant for others. There is a reason of the Baal Shem Tov recorded regarding this matter. [Rebbe in Reshimos Hayoman 4:12 and 22, p. 173; See Hayom Yom Hamevuar p. 419] Practically, the final directive of the Rebbe was to say it quietly, as recorded in the updated printing of Hayom Yom.
Other opinions and customs: Some Poskim rule that the Mizmorim of Shalom Aleichem, Ashes Chayis, Askinu Seudasa, are not to be recited on the night of the Seder in order to be able to begin the Seder right away. [Vayaged Moshe 13:2; See Piskeiy Teshuvos ibid] Others say that the custom is to never recite the above paragraphs on any night of Yom Tov, even if it coincides with Shabbos. [Alef Hamagen 583:1] Otehrs say that the Mizmorim are to be recited regularly [aloud]. [Mateh Efraim 583:1; Likkutei Maharich 3:40; Kaf Hachaim 583:3] Seemingly, the Chabad custom of saying it quietly is a compromise between the various opinions.
Singing Azamer Bishvachin: Some Poskim rule that the song of Azamar Bishvachim is not to be sung when Shabbos coincides with Yom Tov. [Mateh Efraim ibid] Others, however, write that it is to be sung. [See Kaf Hachaim 583:3] Regarding the general Chabad custom of singing these Zemiros-See Sefer Haminhagim p. 29 in the footnotes that it was not always sung by the Rabbeim.
 Seemingly, this is due to what was explained in the previous footnote, that the original Chabad custom was not to say it at all, and thus no mention of it was made in the Rebbe’s Haggadah. However, in accordance to the final directive, one is to say it.
 Admur 473:5; Michaber 473:1
 Siddur Admur; Admur 271:19
 The reason: This is said in order to fulfill the obligation of Vayechulu on behalf of one’s children and household who were not present in Shul and thus did not hear it being said by the congregation after the Davening. [Admur ibid]
 Siddur Admur; See Admur 487:3 regarding Shemoneh and the same applies regarding Kiddush, as brought in M”A 487:2 that Kiddush is even more severe, and M”B 487:2
 See Admur 487:3; M”B 487:2; Piskeiy Teshuvos 473:2; 487:4
 Admur 487:3 regarding Shemoneh Esrei; M”B 487:2 that the same applies for Kiddush
 Admur 487:1 regarding Shemoneh Esrei; M”B 487:2 that the same applies for Kiddush
 This is not considered as if he is adding to the four cups of wine, as he is obligated to say Kiddush, and has not fulfilled his obligation from the current Kiddush recited. [See Seder Hearuch 52 footnote 15]
 Admur 487:3 in parentheses; Rama 487:3; Beis Yosef 487 in name of Orchos Chaim
 2nd opinion and conclusion of Admur 487:3 [that it is proper to not repeat the blessing due to Safek Brachos Levatala] regarding if did not mention Shabbos in concluding blessing but mentioned in middle of Shemoneh Esrei and the same applies for Kiddush; Kneses Hagedola 487; Chok Yaakov 487:4; M”A 487 regarding if mentioned in conclusion but not in middle
Other opinions: Some Poskim rule that if one did not conclude Mikadesh Hashabbos Viyisrael Vihazmanim, which mentions both Shabbos and Yom Tov, then he does not fulfill his obligation of Shemoneh Esrei [or Kiddush] even if he mentioned Shabbos in middle of the paragraph. [1st and Stam opinion in Admur ibid; Elya Raba; M”B 487:2;]