Pores Mapa Umikadeish:
Saying Birchas Hamazon before sunset: When a Seudas Mitzvah falls on Friday one is to eat the meal prior to the 10th hour, and if this cannot be done it may be eaten anytime before sunset. Irrelevant to when the meal was started, it is best to complete the meal and recite Birchas Hamazon prior to sunset. If one did not recite Birchas Hamazon prior to sunset, he may not recite it until he performs Pores Mapa Umikadeish, in the order that will be explained below. [This however only applies if one plans to have a Kos Shel Bracha after Birchas Hamazon, or plans to begin his Friday night meal right away, in which case he cannot recite Birchas Hamazon after sunset until he performs Pores Mapa. If however one does not plan to have a Kos Shel Bracha after Birchas Hamazon, and he does not plan to begin his Friday night meal until later on, then he may even initially stop eating and drinking by sunset, and recite Birchas Hamazon after sunset without first performing Pores Mapa. So was the custom of the Rebbe and Rebbe Rayatz, and so was publicly done by the Rebbe’s Farbrengen on several occasions. See footnote regarding saying the Harachaman of Shabbos in Bentching in such a case.]
The order of Pores Mapa: Pores Mapa Umikadeish means that at sunset one stops eating, brings Lechem Mishna to the table and places a Challah cover over it, recites Kiddush, and eats a piece of bread. The following is the order:
- At sunset one stops eating.
- One brings Lechem Mishna [two whole loaves] to the table and covers it.
- One then says Kiddush over wine. However if one drank wine during the meal, the blessing of Hagafen is not said over the Kiddush. Rather one says Vayechulu and then goes straight to the blessing of Mikadeish Hashabbos. One is then to drink the wine. If there are people listening to the Kiddush which have not drunk wine during the meal, then upon them receiving the wine they are to say a blessing of Hagafen.
- Immediately after Kiddush, without washing hands or saying Hamotzi, one is to cut the bread, and all participants are to eat a Kezayis/Kebeitza size of bread in order to fulfill their Friday night meal. The blessing of Hamotzi is not said over the Lechem Mishna, as one has already said Hamotzi in the beginning of his meal.
- If one planned to recite Birchas Hamazon prior to sunset and has already washed Mayim Achronim, and then sunset arrived prior to him beginning to recite it, then he is to be Pores Mapa as explained above and say a blessing of Hagafen over the wine in Kiddush and a blessing of Hamotzi over the bread.
The Chabad custom on Pores Mapa Umikadesh:
The Rebbe and the Previous Rebbe would not perform Pores Mapa after sunset. They would not eat or drink anything after sunset and would recite Birchas Hamazon later on into the night. Nevertheless the custom by Farbrengen that the Rebbe attended was to perform Pores Mapa. During those gatherings the Rebbe gave specific instructions on different matters in Halacha that the congregation must be aware of. However from 1984 and onwards, due to the Halachic complications involved in following this order, this custom was stopped, and rather the Rebbe would hold the Farbrengen on Shabbos afternoon. Some Chassidim, based on the above, are particular not to perform Pores Mapa.
If one started his meal on Shabbos, Rosh Chodesh etc and ended it into the night which is the start of another Festival or Shabbos is Ritze and Yaaleh Veyavo to be said?
- Rosh Chodesh/Yom Tov/Purim/ Chanukah which is Friday and meal continued into Shabbos.
- Shabbos which is Erev Rosh Chodesh/Yom Tov/Purim Chanukah and meal continued into Rosh Chodesh etc.
One ate a Kezayis past the night: [If the order in Bentching is to first say the previous day and then the night, as on Moitza Shabbos which is Rosh Chodesh that first one says Ritzei and then Yaleh Veayavo then] one is to say the extra prayer of the night, and if did not daven Maariv/Havdala etc then he also says the extra prayer of the previous day. [Thus in above case says both Ritzei and Yaleh Veyavo]. [If however the prayer of the night is said in Bentching prior to the prayer of the previous day then according to all one is to only say the prayer of the night, unless the day is Shabbos or Yom Tov in which case it takes precedence of that of the night.]
[Practically however the Rebbe ruled, and so is the Chabad custom, to say both the prayer of the night and of the previous day even if the night prayer is said first [ie. Yom Tov which continues into Shabbos].] See Q&A regarding Al Hanisim!
Davened Maariv/Havdala before Bentch: If however one Davened Mariv prior to Bentching then the previous days extra prayer is not said, and if one ate a Kezayis only the nights extra prayer is said.  If one did Havdala or even said Baruch Hamavidil it likewise may not be said. [If one said Kiddush, one may nevertheless say the prayers of both days.]
One did not eat a Kezayis past midnight: Only the extra paragraph of the previous day is said, if one did not yet Daven Maariv etc. If one Davened Maariv no extra paragraph is said.
Meal of Shabbos/Holiday/Rosh Chodesh continued into Rosh Chodesh/Holiday/Shabbos: 
One is to always say the prayer of previous day [even if was Pores Mapeh Umikadeish] unless said Havdala or Maariv. One is to also say the prayer of the night if ate Kezayis at night. If oen Davened Maariv and did not eat a Kezayis at night, no extra prayer is said.
Does the above ruling of saying both the day and night prayer apply also by Chanukah/Purim?
· Example 1: Chanukah/Purim fell on Erev Shabbos and one continued his meal into Shabbos
· Example 2: Chanulah/Purim fell on Moitza Shabbos and one continued his meal from Shabbos into them.
Some Poskim rule the above ruling of Admur to say both the day and night prayer applies only to Ritzei and Yaleh Veyavo. However, Al Hanisim is not to be said together with Ritzei, whether Purim/Chanukah fell on Erev Shabbos or whether it fell on Moitza Shabbos. Perhaps however Al Hanisim should be said in Harachaman.
 Admur 271:9-11
 271:11 in end and 271:12; See Chabad custom brought below!
The reason: As if one does not recite Birchas Hamazon before sunset he will be required to begin the next meal and perform Pores Mapa [271:12 and so is implied from end of 271:11] and the performance of Pores Mapa enters one into dispute. Thus one who is meticulous is to avoid it all together. [271:11] The following is the dispute involved: It is disputed whether one has to say another blessing of Hamotzi on the bread that he eats after Kiddush. Some say the saying of Kiddush over wine is considered an interval between the previous blessing of Hamotzi and the bread. Others hold it is not considered an interval and hence a new blessing is not to be said over the bread eaten after Kiddush. Practically we rule leniently when it comes to a questionable blessing. However it is best to avoid the situation all together when possible. [ibid]
 The reason: The reason why one is not allowed to recite Birchas Hamazon past sunset, and must rather follow the order of Pores Mapa, is because if one were to recite Birchas Hamazon after sunset it would enter him into a number of disputes. There would be a question as to whether he is to say a blessing over the Kos Shel Bracha after Birchas Hamazon and then say Kiddush, or if he must first say Kiddush on a separate cup of wine, and is then to drink from the Kos Shel Bracha. Likewise if one says Kiddush right after reciting Birchas Hamazon there is a dispute as to whether he must now eat a meal in order so his Kiddush be recited in the place that he will be eating his meal. Now if he follows this opinion, which he is to suspect for, and eats another meal after Kiddush, when he says Birchas Hamazon for his Erev Shabbos meal it is considered an unnecessary blessing, as he could have not recited an after blessing before Kiddush and hence said one after blessing for both meals. Thus it is better for one to be Pores Mapa and begin his next meal than to recite Birchas Hamazon after sunset and enter these disputes. [271:12]
 One is allowed even initially to recite Birchas Hamazon after sunset without a Kos Shel Bracha and then begin his Shabbos meal later at night, as by doing one does not enter himself into any disputes, as the Birchas Hamazon is then not considered an unnecessary after blessing. This is also implied from the wording of Admur “if he were to say Kiddush immediately after Birchas Hamazon this would enter into the following dispute” thus implying that delaying the Kiddush does not involve any disputes. So also writes Admur explicitly in 271 KU”A 4 that if one delays the Kiddush and Friday night meal there is no prohibition of an unnecessary blessing taking place. So also writes Admur explicitly in 271 KU”A 4 that if one delays the Kiddush and Friday night meal there is no prohibition of an unnecessary blessing taking place. See also Seder 2:4; 168:8; Ketzos Hashulchan 48:20. Thus seemingly there is no restriction for one to recite Birchas Hamazon after sunset if he wishes to not begin his night meal right away. Based on this one can understand the custom of the Rebbe and the Previous Rebbe who would not perform Pores Mapeh after sunset and would rather not eat or drink anything and then recite Birchas Hamazon later on into the night, contrary to the ruling here. One can thus possibly explain that they were not planning to Bentch on the Kos Shel Bracha [requires verification] and also did not plan to eat the night meal right away, and hence circumvented this dispute.
In summary: If one does not plan to have a Kos Shel Bracha after Birchas Hamazon and he does not plan to begin his Friday night meal until later on, then he may recite Birchas Hamazon even after sunset. In such a case it is better for him to recite Birchas Hamazon than to perform Poreis Mapa as doing so involves a dispute regarding the blessing over Hamotzi as explained above.
 The Rebbe and the Previous Rebbe would not perform Poreis Mapeh after sunset and would rather not eat or drink anything and then recite Birchas Hamazon later on into the night. Some Chassidim based on this are also particular not to perform Poreis Mapah. [See Oatzer Minhagei Chabad Rosh Hashana 372; Sichas Simchas Torah 1957; Sefer Hasichos 1944 Simchas Torah footnote 25; Sichas 2nd day of Rosh Hashanah 5744; Sichas Shabbos Bereishis 5744]
 So was done in the Farbrengen of Erev Shabbos, 2nd day Rosh Hashanah in 5748, Erev Shabbos Simchas Torah 1988; See also Frabrengen of Erev Shabbos Rosh Hashanah and Simchas Torah 5741 and 5744
 Regarding reciting Harachaman in Birchas Hamazon in a case that one did not do Pores Mapa after sunset, but is bentching after sunset, the Rebbe stated that it is proper to be said. [See Sichas 2nd day Rosh Hashana 5744; Oatzer Minhage Chabad ibid] However in a later Sicha of Bereishis 5744 the Rebbe limited this saying of Harachaman to only the person leading the Birchas Hamazon, while for other individuals it is evident from that Sicha that it is not to be said. Furthermore, the Rebbe stated that this saying of Harachaman even by the person leading the Bentching is only relevant when part of the congregation has performed Pores Mapah. However when no one has performed Pores Mapah then it is not to be said even by the person leading the Birchas Hamazon. Hence in conclusion it appears that today that we no longer perform Pores Mapah, the Harachaman is not said at all in Birchas Hamazon. So concludes also Otzer Minhagei Chabad ibid. Vetzaruch Iyun.
 The concept of bringing Lechem Mishneh to the table when one performs Pores Mapa is not mentioned in Shulchan Aruch or later Poskim. However so concludes Piskeiy Teshuvos 271 footnote 146 [see there!], and Purim Hameshulash brought in Piskeiy Teshuvos 695 footnote 34. However the Halef Lecha Shlomo 1:113 rules that it is not necessary to have Lechem Mishneh according to the opinion that holds a blessing of Hamotzi is not recited. Vetzaruch Iyun!
 271:11; Sichas 1957 Simchas Torah 3
 As Kiddush is required to be Bemakom Seuda.
 Seemingly one is to eat a Kebeitza of bread in order to fulfill his obligation of Seudas Shabbos according to all. [See Admur 274:6; 639:19 that we mainly rule like the stringent opinion, and so rules Piskeiy Teshuvos 695 footnote 34 that it is proper to eat a Kebeitza. However see Piskeiy Teshuvos 271:14 which rules that one only needs to eat a Kezayis even though in general one needs to eat a Kibeitza of bread during the Shabbos meals. Vetzaruch Iyun.
 This matter of dispute has been discussed in previous footnotes. See there!
 See Seder 5:4-5; 179:5
 Otzer Minhagei Chabad Rosh Hashana 372
 Sefer Hasichos 1944 Simchas Torah footnote 25; See there that in that year Simchas Torah coincided with Erev Shabbos and the Yom Tov Farbrengen continued into Shabbos. The Rebbe Raytaz then sent two Chassidim to look into the law of Pores Mapa. The Rebbe Rayatz then stated that the Rebbe Rashab never performed Pores Mapa with exception to 1920. The following is the story he related: I [The Rebbe Rayatz] used to perform Pores Mapa on many occasions and my father the Rebbe Rashab never told me anything against doing so, until one year he told me it is not befitting to do so for several reasons. From then on I stopped. In that year of 1920 Simchas Torah also fell on Erev Shabbos and the meal continued into Shabbos. Rav Yaakov Landau desired to make Kiddush and perform Pores Mapa, as stated in Halacha. I however refused that my father should make the Kiddush. My father however acquiesced to Rav Yaakov Landaus request and agreed to make Kiddush. I then stated my opinion and my father proceeded to send me on an errand in order so I am not present while Kiddush is performed. He also told me that even an older son must listen to his father. I hence went, due to lack of choice, to do the errand. On my way I hid the wine to prevent them from making Kiddush. However Rav Yaakov Landau out of his great stubbornness, finally managed to find wine elsewhere and brought it to the table for my father to make Kiddush, as it states in Halacha. When I returned from the errand and noticed that Kiddush was made I was overcome by emotion and ill feeling. [Sefer Hasichos ibid]
 See Sefer Haminhagim p. 120 [English] that the custom of Pores Mapa is recorded; Sichas Simchas Torah 1957 for instructions on Yaaleh Veyavo and Ritzei in Birchas Hamazon. The Rebbe would customarily hold a Farbrengen on the 2nd day of Rosh Hashana, towards evening, which would include a Mamar that mentioned all the Rabbeim and was followed by Kos Shel Bracha. In years that the conclusion of Rosh Hashana coincided with Shabbos, up until 1984, the Rebbe would hold the Farbrengen as usual, on the 2nd day of Rosh Hashana which was Friday afternoon. The Farbrengen would enter into Shabbos and Pores Mapeh Umikadesh was performed. In such a case the Kos Shel Bracha would be delayed until Motzaei Shabbos. However from 1984 and onwards, due to the Halachic complications involved with this procedure, this custom was stopped, and the Rebbe would rather hold the Farbrengen on Shabbos afternoon. The Rebbe would tell all the Chassidim to wash for bread at the start of the Farbrengen.
 As since one says the prayer of the day first it is found that one has not done any matter which contradicts his statement of the previous day. [So rules Taz]
However others [Magen Avraham] argue that as soon as night begins one can no longer say both the prayer of the previous day and the extra prayer of that night due to it being a contradiction. Rather one is to say the prayer of the night [as according to all the prayer of night must be said, while it is a dispute if the prayer of day must be said, as explained above, and one cannot say both as this is a contradiction.] If however the day is Shabbos or Yom Tov then its prayer is to be said rather then that of the night, as Ritzei and Yaleh Veyavo of Yom Tov is an obligation. Practically the custom is like the first opinion. [ibid]
 As according to all the prayer of night must be said, while it is a dispute if the prayer of day must be said, as explained above, and one cannot say both as this is a contradiction. [ibid]
 So rules also Ketzos Hashulchan 47 footnote 22
 Shaareiy Halacha Uminhag p. 211; Sefer Hasichos 5704 p. 40
 As it is a contradiction to have prayed the prayer of the next day and then mention the previous day.
 Shaar Halacha Uminhag p. 211; Sefer Hasichos 5704 p. 40; So is also proven from Ketzos Hashulchan 47 footnote 22 that Kiddush makes no relevance in the question of if one may say the previous days prayer.
 188/17; Shaar Halacha Uminhag p. 211; Sefer Hasichos 5704 p. 40
Ruling of Admur in the Shulchan Aruch [ibid]: If the nights extra prayer comes before the days extra prayer in Bentching then one cannot say both prayers. Rather say the nights unless the days is more important in which case only say the days.
 Ketzos Hashulchan 47/10-11
 As in such a case it is proper to suspect for the opinion of the M”A that both prayers are never said, being that Al Hanisim is not an obligation to mention, as well as that there are opinions which hold we never say the Alk Hanisim for the previous day even when there is no contradiction.
 As in such a case one cannot say both [according to ruling of Admur, even according to Taz] being that Al Hanisim is said before Ritzei of previous day. [This would apply even according to our Minhag based on directive of previous Rebbe, as explained in the previous footnote.