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The Baal Shem Tov was accustomed to eating three meals on the last day of Pesach. This third meal was referred to as Moshiach’s Seudah, as on this day shines a revelation of Moshiach. This concept is relevant to everyone of the generation.
When is it celebrated? One celebrates Moshiach’s Seudah on the last day of Pesach as the third meal of the day, after Mincha. In Eretz Yisrael, it is celebrated on Shevi’i Shel Pesach, while in the Diaspora it is celebrated on Acharon Shel Pesach. In Eretz Yisrael, when Shevi’i Shel Pesach coincides with Erev Shabbos, one is to celebrate Moshiach’s Seudah on Erev Shabbos prior to the 10th hour of the day, making sure to complete the meal before sunset, although one may recite Birchas Hamazon after sunset. In Eretz Yisrael, one is likewise to continue the celebration on Shabbos day [through making another Moshiach’s Seudah that next day]. Everyone must participate in the Seudah and is not to set himself apart from the community.
The Menu: One is to drink four cups of wine. Upon drinking each cup, one is to have in mind their connection to the future redemption. One who drank without this intent is to re-drink that cup of wine.
Is one required to have Lechem Mishneh by Seudas Moshiach?
Some Poskim rule one is required to have Lechem Mishneh during every meal of Yom Tov, even in one’s third or fourth meal, just as is the law by Shabbos. [Practically, it is proper to be stringent in this matter.]
 Sefer Haminhagim p. 84 [English]; Likkutei Sichos 4:1298; In Sichas 5702 p. 109-110 the Rebbe Rayatz referred to the meal as Seudas Acharon Shel Pesach, and not as Seudas Moshiach. However, in Sefer Hasichos 5700 p. 75 the Rebbe Rayatz brings in the name of his father, in the name of the Rebbe Maharash that it was called Seudas Moshiach. See also Sefer Hasichos 1703 p. 118; Otzer Minhagei Chabad p. 229-230
 Sefer Haminhagim ibid; Hayom Yom p. 47
The reason: The reason this day contains a revelation of Moshiach is because on Shevi’i Shel Pesach the souls are born, and Moshiach will only come after all the souls have descended into this world. [Sichas 1938 p. 275]
 Hisvadyus 5748, page 171
 Sefer Haminhagim p. 83
 Admur 529:2; To note, however, that by the Rebbe there were times that the Yom Tov Farbrengen even initially began past the 10th hour of the day of Erev Shabbos.
 Admur 271:9-11
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. [Admur ibid]
 Hisvadyus 1988 p. 76
 Hisvadyus 5749 page 57
 This custom began in 1906 as a directive of the Rebbe Rashab to the students of Tomchei Temimim on Acharon Shel Pesach of that year drink 4 cups of wine during this meal. [Hayom Yom Mamar Vehichrim 5749] Although this custom was originally viewed as one-time occurrence related to that year, it became widespread in later years during the Rebbe’s Nessius, as the Rebbe requested for each person to fulfill it. [Otzer Minhagei Chabad p. 232]
 In 1906 the Rebbe Rashab stated that the 4 cups drank on the first night of Pesach correspond to Moshe Rabbeinu, while the 4 cups of Acharon Shel Pesach correspond to Moshiach. [Otzer Minhagei Chabad p. 232]
 Mamar “Vehichrim” Melukat 3 p. 188
 Otzer Minhagei Chabad p. 231
 Sichas 1944 p. 116;
The reason: The Shpolo Zeida was involved in redeeming captives. So too on Pesach we have all been redeemed. [See Otzer p. 237]
 Otzer Minhagei Chabad p. 237
 Hisvadyus 3 p. 62
 Kaf Hachaim 529:21; SSH”K 55:1-2 and footnote 6; Piskeiy Teshuvos 274:1; Likkutei Sichos 36 p. 78 [brought in Shulchan Menachem 2 p. 75]
The law by Shabbos: In Rama 291:4 and Admur 291:7 a dispute is recorded regarding whether one is obligated to perform Lechem Mishneh each time he has a meal on Shabbos. Some Poskim [Maggid Mishneh; Abudarham] rule one is obligated to perfor4m Lechem Mishneh by every meal of Shabbos, even if he is eating a fourth meal. Others however rule there is never a requirement to have Lechem Mishneh by other meals, even on Shabbos. The Rama and Admur conclude that although the custom is to be lenient nevertheless every person is to be stringent to take Lechem Mishneh. The above Poskim extend this law also to Yom Tov.
Difficulty with above ruling: The above ruling of the Rama has not been recorded in Shulchan Aruch or any of its commentators to apply also to Yom Tov. Likewise, the Rama and Admur [and all other Poskim who record this ruling] specifically write that this law applies to all the meals of “Shabbos”. If this law extends likewise to Yom Tov then why did they specify Shabbos in their ruling? This is especially in light of the fact that it is plainly evident to all that the law is dealing with Shabbos. Hence perhaps it is coming to exclude Yom Tov. Vetzaruch Iyun!
The Rebbe in Likkutei Sichos: In Likkutei Sichos ibid the Rebbe clearly writes that according to the opinion that requires Lechem Mishneh by each meal it applies equally to Yom Tov. Likewise, according to the reason of Betzias Hapas [see next footnote], which applies by Yom Tov, one requires Lechem Mishneh by every meal. [Nevertheless, it remains questionable as to whether the Rebbe’s intent is to rule this way practically regarding Yom Tov, as even regarding Shabbos a dispute is recorded regarding this matter, and perhaps the purpose of the Sichah was to explain the background of the matter]
 The reason: In Likkutei Sichos ibid the Rebbe explains that there are two reasons behind Lechem Mishnah; one is due to the obligation of the Shabbos and Yom Tov meal and another is due to the laws of Betzias Hapas. This means that due to the importance of Shabbos and Yom Tov not only is one to say Hamotzi on a Shaleim, as is the ruling even during the week, but one is to have two Shleimus. Now, this law of Betzius Hapas applies to all meals that one has on Shabbos and Yom Tov and is not limited to the meals of obligation. This is one of the practical ramifications between the two reasons behind Lechem Mishneh.