Davening Mincha past sunset:
The prayer of Mincha may be recited until evening. It is disputed amongst Poskim as to the definition of “evening.” Some Poskim rule it is permitted [even initially] to Daven Mincha past sunset, up until Tzeis Hakochavim. Other Poskim rule one may not Daven Mincha past sunset. Practically, one is to initially Daven Mincha before sunset [and so was the initial directive and custom of the Chabad Rabbeim who were very careful in this matter], although one is not to protest against those who are lenient to Daven Mincha after sunset, until Tzeis Hakochavim. This applies both on Shabbos and during the week. This especially applies, in a time of need, such as in the short winter days, [that one may be lenient to Daven Mincha after sunset, up until nightfall].
Initially, one is to Daven Mincha before sunset, although one is not to protest those who are lenient to Daven Mincha after sunset, until Tzeis Hakochavim. This applies both on Shabbos and during the week. In a time of need, such as in the short winter days, one may be lenient to Daven Mincha after sunset, up until nightfall.
Is it better to Daven Mincha in private before sunset than to Daven with a Minyan after sunset?
Some Poskim it is better to Daven Mincha in private before sunset than with a Minyan after sunset. Other Poskim however rule it is better to Daven Mincha with a Minyan after sunset, then to Daven in private beforehand. It is unclear from Admur as to his opinion on this matter.
The Chabad custom: Although many are accustomed like the latter approach to pray after sunset for the sake of Davening with a Minyan, practically the evidence points that the Chabad custom should follow the former approach to Daven Mincha in private before sunset rather than with a Minyan after sunset as so was done by the Rebbe Rashab and Rebbe Rayatz and Rebbe, as well as vintage Chabad Chassidim, and so was the directive of the Rebbe Rayatz and the Rebbe to an individual
Practically, however, this latter approach should only be followed by the individual if it will not cause the Shul Minyan to be nullified.
Is one to initially Daven Mincha with enough time to complete Shemoneh Esrei before sunset?
May one Daven Mincha past Tzeis Hakochavim?
 See Mamar Rebbe Maharash 5627 p. 52; Piskeiy Teshuvos 233:6; Biurim on Seder Hachnasas Shabbbos [Levin] p. 65-67; Halacha Berura [Rav Dovid Yosef] 233
 Chachamim in Mishneh Brachos 26a and Michaber 233:1, based on Talmidei Rabbeinu Yona on Rif Brachos ibid, that the custom is like Chachamim; See also Shut Admur 7 “And in these generations, many are stringent to Daven Maariv on time”; See Tzemach Tzedek Chidushim on Brachos ibid
Opinion of Rebbe Yehuda: Rebbe Yehuda in the Mishneh ibid famously ruled that Mincha may only be prayed until Plag Hamincha. Thus, the question of Davening Mincha past sunset is only relevant to the opinion of the Chachamim. Practically, the Michaber concludes that the custom is like Chachamim and so is the widespread custom today.
 Implication of Michaber ibid “Until nighttime according to Chachamim..And now that the custom is to Daven Mincha until nighttime” [so implies Aruch Hashulchan 233:8 and M”B 233:14, unlike his interpretation in 233:8]; Rama ibid “He is Yotzei if he Davens Mincha until nighttime, which is until Tzeis Hakochavim”; Rashi Brachos ibid “until night”; Implication of Hagahos Maimanis Tefila 3 Halacha 4 Os 3 in name of Rabbeinu Chananel; Shaagas Aryeh 17; P”M 89 M”Z 1 “Bein Hashmashos is also a time of Mincha for Chachamim”; 106 M”Z end of siman “Mincha can also be prayed during Bein Hashmashos” [contradicts P”M 233 A”A 7]; Implication of Gr”a 233 [unlike Gr”a 261, see Shaar Hatziyon 233:18]; Ashel Avraham of Buchach 233 “The time of Mincha is until Tzeis Hakochavim”; Maharam Shick 91; Mamar of Rebbe Maharash ibid; Rav Poalim 2:19 based on Poskim and Mekubalim and that so is custom in Yerushalayim and Chevron; and Ben Ish Chaiy Vayakhel 7 regarding women; Kaf Hachaim 233:4-5; Eretz Tzevi 1:1; Shut Beis Avi that so is custom of Polish Jewry to even initially Daven Mincha past sunset; Minchas Elazar 1:13 and Nimukei Orach Chaim 131:2 that the custom of the Belzer Rav was to delay Mincha until close to Tzeis Hakochavim; Meumei Sadeh on Brachos ibid [of Nesiv]; Divrei Yatziv 98-99 that so was custom of Tzanzer Rebbe, Chasam Sofer and Reb Akiva Eiger and so is agreed to by more than 40 Rishonim and Achronim; See Aruch Hashulchan 233:8; Or Letziyon 1:20; Yechaveh Daas 5:22; Yabia Omer 7:34; Mishneh Halachos 7:32; Halacha Berura [Rav Dovid Yosef] 233 for list of many Rishonim and Achronim
Chabad sources: The following Sefarim rule or imply like this opinion that one may pray Mincha past sunset, until nightfall. [Mamar of Rebbe Maharash ibid that according to Chochamim it is because it corresponds to the incense; Mamar of Rebbe Rayatz, printed in Kuntreisim 1:407 and Sefer Hamamrim 5691 p. 347 that Pelag Hamincha is an hour and 15 minutes prior to nighttime.]
 So is implied from wording of Poskim ibid, and so is explicitly written in some Poskim ibid; Now, although the Rama permits only Bedieved and time of need, this is because he was referring to those who follow Rebbe Yehuda on a normal basis, however for those who always follow Chachamim, he too would agree that even initially it is allowed
 The reason: Perhaps the reason is because Mincha is Rabbinical, and the Sages permitted reciting it during Bein Hashmashos, due to Safek Derabanan Lihakel. [Aruch Hashulchan ibid; see Admur in Seder Hachnasas Shabbos] Alternatively, the reason is because Mincha corresponds to the Ketores and not to the afternoon Tamid, and the Ketores was allowed to be offered until Tzeis Hakochavim. [Hagahos Maimanis Tefila 3 Halacha 4 Os 3 in name of Rabbeinu Chananel; Rebbe Yossi in Yerushalmi Brachos 4:1 based on Tehillim 141:2; Aruch Hashulchan ibid; Mamar of Rebbe Maharash ibid; See Biur Hagr”a ibid;] Alternatively, in truth the limbs of the afternoon Tamid could be offered until Tzeis Hakochavim. [Pnei Yehoshua Brachos 27a; Shaagas Aryeh ibid] The Talmidei HaBaal Shem Tov would intentionally delay Mincha until close to night, in order to battle the Kelipos of darkness with their prayers. [Nimukei Orach Chaim ibid]
 Implication of Rambam Tefila 3:4 “until sunset” [see; Talmidei Rabbeinu Yona on Rif Brachos ibid “Do not interpret evening to mean until nightfall but rather until sunset”; Shiltei Hagiborim on Mordechai; Kneses Hagedola 233; Lechem Chamudos; Levush 233; Gr”a 261 and in Maaseh Rav 64 [unlike Gr”a 233, see Shaar Hatziyon 233:18]; P”M 233 A”A 7 “Until Tzeis Hakochavim is difficult to allow” [contradicts P”M 89 M”Z 1; 106 M”Z end of siman]; Aruch Hashulchan ibid concludes to be stringent like Rambam and not Daven Mincha after sunset and to protest strongly against those who do; M”B 233:7 and 14 interprets Michaber and Rama ibid “until night” and “until Tzeis Hakochavim” to mean “until the beginning of Bein Hashmashos” however he then writes that the simple implication is until Tzeis Hakochavim. The M”B ibid concludes to be stringent like the above Poskim to be very careful to Daven Mincha before sunset and that only Bedieved and in a great time of need may one be lenient, and even then only until 15 minutes before Tzeis Hakochavim; See Halacha Berura [Rav Dovid Yosef] 233 for list of many Rishonim and Achronim
Chabad sources: The following Sefarim rule or imply like this opinion that one must Mincha before sunset. [Mamar of Rebbe Rayatz, printed in Kuntreisim 1:407 and Sefer Hamamrim 5691 p. 347; Igros Kodesh Rayatz 3:250; Likkutei Dibburim 1:79 that so did Rebbe Rashab that he Davened Beyechudius before sunset, rather than delay until later, and so followed the Chassidim Rav Yaakov Landau and Rav Yitzchak Dubov]
Does the above opinion follow the opinion of Geonim or Rabbeinu Tam? See M”B ibid who is clearly going on the opinion of Rabbeinu Tam, and so can be interpreted in many of the above Poskim. See Piskeiy Teshuvos 233:8 and Halacha Berura ibid. Accordingly, it is possible to learn in many of the above Rishonim and Poskim, that they only argued against Davening Mincha past the sunset of Rabbeinu Tam, however, by the sunset of the Geonim, perhaps they agree that one may Daven past sunset. However, in truth, Admur in the Siddur learns the above Poskim to refer to even sunset of the Geonim, as stated there.
 The reason: As the prayer of Mincha was instituted in correspondence to the afternoon Tamid sacrifice, whose blood could only be offered until sunset. [Talmidei Rabbeinu Yona on Rif ibid]
 Likkutei Dibburim 1:79 that so did Rebbe Rashab that he was particular to Daven Beyechudius before sunset, rather than delay until later, and so followed the Chassidim Rav Yaakov Landau and Rav Yitzchak Dubov; Hearos Ubiurim Ohalei Torah 963 p. 152 [Rav Label Shapiro]; 964; 971 p. 64; 975 p. 65; 1073 p. 89 [Rav Ginzberg]; Hiskashrus p. 65; Heichal Menachem 3:235
 Admur in Seder Hachnasas Shabbbos “However regarding Mincha, which is Rabbinical, one is not to protest against those who are lenient whether during the week or on Shabbos, especially in a time of need during the short winter days.”; Ketzos Hashulchan 26:1 “Mincha may be prayed until sunset, although Bedieved one is Yotzei until Tzeis Hakochavim”; All Poskim brought in coming footnotes who say in time of need may Daven after sunset
 Admur ibid
 Poskim in next footnote; Seemingly, the intent of Admur is that even initially one may Daven Mincha during Bein Hashmashos in a time of need, and not that simply one is not to protest those who are lenient in a time of need. VeTzaruch Iyun. See Biurim ibid p. 66
 Siddur Admur in Seder Hachnasas Shabbos; Darkei Moshe 233 that Maharil would Daven Mincha close to Tzeis Hakochavim in a time of need, brought in M”A 233:6; Mor Uketzia 233; Shoel Umeishiv Gimmel 247; Kitzur SHU”A 69:2; Ketzos Hashulchan 26:1; Or Letziyon 1:20; Shevet Halevi 9:48; M”B ibid also permits in time of need however he limits this to a case of “great need” and only until 15 minutes before Tzeis [according to Rabbeinu Tam]; Piskeiy Teshuvos ibid
The reason: As we see that the Sages permitted one to perform Rabbinical Melacha during Bein Hashmashos of Friday in a time of need, and for the need of a passing Mitzvah. The same then applies regarding Mincha, which is only a Rabbinical obligation. [Admur ibid; See Admur 261:2]
 M”B 233:14
 Mor Uketzia 233; Shoel Umeishiv Gimmel 247; Shearim Hametzuyanim Behalacha 69:1; Or Letziyon 1:20; Shevet Halevi 9:48; Eiynayim Lamishpat Brachos 27a; Piskeiy Teshuvos 233:6; Sichas Hashavua Gilyon 1117
 Admur does not make clear as to what is defined as a time of need to allow delaying Mincha until after sunset. See Admur 90:11 that Davening within the Zeman is more important than Davening with a Minyan.
 See Hearos Ubiurim Ohalei Torah 963 p. 152 [Rav Label Shapiro]; 964; 971 p. 64; 975 p. 65; 1073 p. 89 [Rav Ginzberg]; Hiskashrus p. 65
 As apparent from the story in Likkutei Dibburim 1:79 that the other Chassidim did Daven after sunset with a Minyan and as testified by the person in Yechidus to the Rebbe in Hiechal Menachem ibid; Sichas Hashavua ibid
 Likkutei Dibburim 1:79 that so did Rebbe Rashab that he Davened Beyechudius before sunset, rather than delay until later
 Rav Zalman Shimon Dworkin relates that he personally witnessed that the Rebbe Rayatz was also stringent in this matter and once Davened Beyechidus before sunset during the Shiva for his father rather than wait for the 10th man, and that so he would direct those who would ask him. [Hearos Ubiurim Ohalei Torah 971 p. 65; 975 p. 65; 1073 p. 89]
 When he would return from the Ohel late, he would stop by the side to Daven Mincha before sunset, even though it was Beyechidus [See Hearos Ubiurim Ohalei Torah 963 p. 153]
 Rav Yaakov Landau and Rav Yitzchak Dubov; See Hearos Ubiurim Ohalei Torah 964:82 [Testimony of Rav Chaim Rappaport] and 1073 p. 89
 See Hearos Ubiurim vol 975 p. 65
 See Heichal Menachem 3:235 that in reply to a question of an individual in Yechidus the Rebbe replied of whether he should follow the opinion of the Mishneh Berurah ibid, that he should continue with his custom of following the Mishneh Berurah to Daven Beyechidus before sunset, unlike the common Chassidic custom; Hearos Ubiurim Ohalei Torah 963; 964; 971; 975; 1073 p. 89; Hiskashrus p. 65
 See Hearos Ubiurim Ohalei Torah 964 p. 81-82 and 971 p. 65
 M”B 233:14
 Pashut! See Michaber 233:1; Admur in Seder ibid
 See Taamei Haminhagim in Sefer Zichron Tov; The Rebbe Davened Mincha with a Minyan after nightfall during many of the days between the 19th of Kisleiv 5746 and 8th of Shevat