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When during the day is the Purim meal to be eaten? 
Customarily, the Purim meal is eaten after an early Mincha. One is to beware to eat the majority of the meal while it is still day. [However there are Poskim who justify those that are accustomed to begin the meal later in the day and thus eat the main part of the meal at night. Practically, the Chabad custom is to eat the meal after an early Mincha, as stated above, and not delay the meal until close to sunset.]
Eating the meal in the morning: One fulfills his obligation even if he eats the festive meal in the morning, and one who desires may do so [even initially]. [Furthermore many Poskim rule one is to specifically eat the meal in the morning. Practically, the Chabad custom is to eat the meal after an early Mincha, as stated above, and not specifically in the morning.]
Purim that falls on Erev Shabbos: When Purim falls on Erev Shabbos the meal is to take place in the morning [prior to midday] due to Kavod Shabbos. [This applies even according to Admur which generally only requires a Seudas Mitzvah to take place prior to the 10th hour.] If one is unable to set the meal for the morning, then at the very least it is best to start the meal before the 10th hour of the day. [Some are accustomed to do so even initially.] If this too is not possible, one may start the meal up until sunset. In such a case, if one is unable to eat the Friday night meal, he is to eat three meals on Shabbos day. Regarding continuing the meal into Shabbos-See Halacha 6.
Until what hour is the meal to continue?
The Mahril would continue his meal throughout the night. The Terumos Hadeshen would only recite Birchas Hamazon at 2:00-3:00 in the morning. The Rama writes that the custom was to continue the meal in various homes throughout the night and then recite Birchas Hamazon in the last house. The Rebbe stated that one is to have a large and lively community Farbrengen after the Purim Seuda.
 Rama 695/2
 Rama 695/2; Sefer Haminhagim p. 172 [English]
The reason: The reason for delaying the meal until after Mincha is because in the morning most people are very busy with Mishloach Manos, Matanos Laevyonim, and reading Megillah and hence they do not have time to properly celebrate the meal. [Terumos Hadeshen 110]
 Those which wash close to sunset and then eat the main part of the meal at night after Purim are mistaken. [Rama ibid] One who eats the entire Seuda at night does not fulfill his obligation. [See Kaf Hachaim 695/8]
 Eretz Tzevi 1/121; Minchas Elezar 2/5; Darkei Chaim Veshalom 854; Shut Rama 132/5 [custom is to begin the meal close to night]; See Shulchan Menachem 3/322
The custom of the Rebbe: In Toras Menachem [25/138; Sichas Purim 1959 chapter 44] it is written that the Farbrengen on Purim started late during the day and took place mainly on the night of the 15th and not during the day of the 14th. However perhaps another meal was already eaten during the day, and on this it was relied upon to not require the Farbrengen to take place mainly by day. See also Sichas Purim 1985 in which the Rebbe explained the reason the Farbrengen begins at a late hour is in order so everyone has time to eat a meal and fulfill the Mitzvah of “Ad Delo Yada” with their family. [Brought in Otzer Minhagei Chabad 204]
Having a small meal earlier on in the day: The Mishmeres Shalom 50/5 writes that in truth they would eat a first meal early on in the day and then they would eat a second meal late in the day, and at times after the day was over. Hence the Mishmeres Shalom certainly does not justify having the only meal of the day very close to sunset. This is unlike the ruling of Piskeiy Teshuvos 695 footnote 25 which brings the Mishmeres Shalom to defend such a practice.
 The reason: As since they began the meal while still day the entire meal goes after the daytime. [Eretz Tzevi ibid] Alternatively it is because there is not enough time during Purim day to properly eat the meal and hence the main meal takes place at night, and on the 15th there is a Mitzvah to celebrate just like on the 14th. [Minchas Elazar ibid]
 Sefer Haminhagim ibid; See previous footnotes in custom of Rebbe!
 Terumos Hadeshen 110; Shlah 261a; Elya Raba brought in P”M 695 A”A 5; Maaseh Rav 238; Siddur Harashash based on Kabala; Kaf Hachaim 695/23
 Sefer Haminhagim ibid
 Rama 695/2
 M”B 695/10; Aruch Hashulchan 249/7; See however Shulchan Menachem 3/330 that the Rebbe once stated one is not required to be particular to begin prior to midday, and that the Rebbe Rashab had his meal in the afternoon on Erev Shabbos. [Sefer Hasichos 5700 P. 37]
 Ketzos Hashulchan 69 footnote 8, as by the Purim Seuda there is a greater chance of getting drunk.
 Yad Efraim in name of Mahril brought in M”B 695/10
 Minhagei Eretz Yisrael in name of Nehar Mitzraim; Piskeiy Teshuvos 695/6 footnote 31 in name of Meiri Kesubos 7a that so is the custom in Jerusalem to continue the meal into Shabbos and perform Pores Mapa, although he concludes that based on Kabala it is improper to do so, as one is to Daven Maariv before Kiddush. [See Kaf Hachaim 271/22 in name of Arizal]
 Admur 249/6; Hisorerus Teshuvah 2/172; Piskeiy Teshuvos 695/6 brings from M”B 529/8 that in such a case one is to only eat a minute amount of food. However no mention of this is made in Admur 249 or 529.
The reason: The reason for this is because a Seudas Mitzvah which has fallen out to be eaten specifically on Erev Shabbos, may be set on Erev Shabbos, even if due to this one will be unable to fulfill the Friday night meal, as this meal is likewise a Seudas Mitzvah just as the Friday night meal. [ibid]
When Purim falls on Erev Shabbos and one did not start his meal prior to Plag Hamincha, may he already accept Shabbos and then begin his meal? One is not to do so. [Hisorerus Teshuvah 2/172]
 Admur 249/6
 See Shulchan Menachem 3/322
 Minhagei Mahril Purim
 Leket Yosher 158
 Shut Rama 132/5
 Hisvadyus 1988 2/410