This Halacha is an excerpt from our Sefer
Eating a large meal prior to Mincha and the Seudas Hamafsekes:
It is an accustomed Ashkenazi practice to eat a set meal on Erev Tishe Beav, prior to Davening Mincha. After the meal, one Davens Mincha and then eats the Seudas Hamafsekes. [Some Poskim rule this meal is to be eaten prior to midday. However, the widespread custom is not to be particular in this matter. Some are accustomed not to eat two meals, one before Mincha and one afterwards, and rather they eat one meal with many uncooked foods and then at the conclusion of the meal they eat the egg and bread with ash.]
How much is one to eat by the first meal? The custom is to have a larger meal than usual in the first meal eaten [prior to Mincha]. Nevertheless, one who is able to withhold himself from increasing in food even during this meal and knows that the fast will not damage him, then if he is stringent to do so he is considered holy and praised. [However, other Poskim validate the increasing of food even in such a case.]
Being careful not to satiate oneself too much by this meal: One must beware not to satiate himself too much during the first meal in order so he retains an appetite to eat the Seudas Hamafsekes.
Tachanun: Tachanun is omitted during Mincha of Erev Tishe Beav being that Tishe Beav is called a “Moed” [Festival].
It is customary to eat a large meal prior to Davening Mincha on Erev Tishe Beav. After the meal, one Davens Mincha and then eats the Seudas Hamafsekes.
If one is unable to Daven Mincha with a Minyan after his set meal, prior to the Seudas Hamafsekes, what is he to do?
Is it better to Daven Mincha with a Minyan before the meal eaten prior to Seudas Hamafeskes or should one eat the set meal and then Daven Mincha privately prior to the Seudas Hamafsekes?
If one is eating the two meals close together, then he must Daven Mincha between the two meals. Thus, in the above case, it is better to Daven privately between the two meals than to Daven with a Minyan before the first meal. If, however, in any event there will be a large interval between the two meals, then he may Daven Mincha with a Minyan prior to either meal.
Laws pertaining to the final meal:
What is defined as the Seudas Hamafsekes?
The Seudas Hamafsekes is the last meal eaten before Tishe Beav, after midday, and it is this last meal eaten after midday that contains all the restrictions that apply to the meal due to mourning, as will be explained. All the other meals eaten prior to the last meal do not carry any of these restrictions, even if it is eaten after midday. Likewise, even the last meal itself only carries the restriction when it is eaten after midday. If, however, one eats the last meal before midday it does not carry any of the restrictions.
The menu restrictions-Letter of law:
Below are all the menu restrictions applicable towards the meal of Seudas Hamafsekes, according to the letter of the law. However, the custom is to further restrict the menu of this meal, as explained in C. Thus, idealy one is to follow the accustomed menue brought in C, although in a time of need that he is still hungry, he may follow the letter of the law menu brought below.
Not to eat meat, wine or two cooked foods: It is forbidden from the letter of the law to eat meat, drink wine, or partake in two cooked foods during this meal.
The definition of a cooked food: All cooked foods are included within this prohibition of eating two cooked foods even if it is possible for the foods to also be eaten raw.
Two cooked foods in one dish: Two foods that have been cooked together in the same pot have the status of two cooked foods [and is thus forbidden to be eaten by the Seudas Hamafsekes]. However, if this combination of foods is the most common recipe for this food throughout the year, then it is considered one food, and it may be eaten. Thus, one may eat a cooked legume that contains onions if this is the common recipe of cooking throughout the year.
Raw fruits and vegetables: One may eat an unlimited amount of raw [not cooked] fruits and vegetables during this meal.
Diminish in drinking: One is to diminish the amount he drinks during this meal in comparison to the amount he drinks in a regular meal during the year.
May one drink coffee and tea during the Seudas Hamafsekes?
The accustomed menu:
Bread, salt, and egg: Those who are able should only eat bread with salt [the cold hardboiled egg] and drink water during this meal. [This is the widespread custom amongst those who eat a set meal before Mincha, as explained in the previous Halacha. However, some are accustomed not to eat two meals, one before Mincha and one afterwards, and rather they eat one meal with many uncooked foods, and then at the conclusion of the meal they eat the egg and bread with ash.]
Sitting on the floor during the meal: The custom is to sit on the ground while eating the Seudas Hamafsekes meal. [However, based on Kabala, one is never to sit directly on the ground and is rather to have an interval between him and the floor. Clothing that one is wearing is not considered a valid interval for this matter. One should be stringent even regarding a tiled floor.]
Not to eat in the forum of three men: One should be careful not to eat the meal with another two men in order to avoid the obligation of making a Zimun. [Nevertheless, in the event that one ate with two other men, a Zimun is not to be made.]
Eating after Bentching:
After one finishes his final meal, and recites Birchas Hamazon, he may continue to eat and drink [those foods permitted during the Seudas Hamafsekes] until sundown/Shekia. However, if one explicitly said that he is accepting the fast after his conclusion of the meal, then he may no longer eat and drink [and all the prohibitions that begin by sundown apply to him from that time, other than the prohibition against wearing leather shoes]. This acceptance is valid even if it was not verbalized but simply stated in one’s mind. 
Saying that one is not yet accepting the fast prior to finishing eating: It is proper to stipulate during the Seudas Hamafsekes that even after he finishes eating he does not intend to accept the fast [until sundown and] he may thus continue to eat and drink. [This stipulation is valid whether it is said in the heart, or verbalized.]
If one does not feel like eating anymore: Even if one is satiated and does not feel like eating anymore food, it is not considered an acceptance of the fast until he explicitly decides to begin the fast.
The widespread Ashkenazi custom is to eat a large meal on Erev Tishe Beav prior to Mincha and then eat the Seudas Hamafsekes after Mincha. Customarily, the Seudas Hamafsekes consists of merely bread with salt, water, and a cooked egg. One dips the bread [and egg] in ash and states “This is the meal of Tishe Beav”. One is to sit on the floor [with an interval between him and the floor] during this meal and it is not to be eaten with a Zimun. One is to diminish in his normal amount of drinking during the meal.
Is one to initially recite Birchas Hamazon before sunset?
One may recite Birchas Hamazon even after sunset.
May one perform Mayim Achronim after sunset?
Is one to recite Birchas Hamazon with Nachem if he is reciting Birchas Hamazon after nightfall?
No. One only recites Nachem in Birchas Hamazon on Tishe Beav, if he ate on Tishe Beav itself, such as a child, or one who is sick.
Is one to dip the egg in ash?
The Poskim mention dipping the bread in ash and not the egg. Many however today are accustomed to dip the egg in ash [rather than the bread].
 Rama 552/9
The custom of Sefaradim: The ruling here follows the custom of Ashkenazi Jewry. However, Sephardic Jewry is not accustomed to have a large meal prior to Mincha, prior to the final meal [Seudas Hamafsekes]. [See Michaber 552/9]
 Rama ibid in name of Maharil
The reason: The reason for eating a meal prior to the Seudas Hamafsekes, is because one’s menue is exteremely limited during the Seudas Hamafsekes, and thus in order to prevent the fast from causing one damage [due to hunger] aone should have a first meal in which he can properly satiate himself. [See Rama ibid] Alternatively, the reason for this is to commemorate that in the times of the second Temple, Tishe Beav was a festive day in which a festive meal was held. By this we express our belief that once more Tishe Beav will become a day of rejoicing. [M”A 552/11; Elya Raba 552/11]
 Rama ibid in name of Maharil
The reason: The reason for specifically Davening Mincha between the two meals is because this creates an interval and prevents the issue of causing a Bracha Sheiyno Tzricha. [Shaar HaTziyon 552/16] The Darkei Moshe records in the name of the Mahril that one goes to the Beis Midrash and Davens Mincha. [M”B 552/21]
 M”A 552/10; Darkei Moshe 552/3; Bach; Ateres Zekeinim 552; Kaf Hachaim 552/46
 The M”B 551/20-22 does not mention at all the need to eat the meal before midday. It is also omitted from Piskeiy Teshuvos 551/8 and Nitei Gavriel 51/1-2
 Piskeiy Teshuvos 552/8
 Rama ibid; Darkei Moshe 552/3 [brought in Kaf Hachaim 552/48] “This is the custom of majority of people”
Other opinions: Many Poskim argue and rule that it is forbidden to increase in eating during the first meal to the point one no longer desires to eat the Seudas Hamafsekes and simply eats it because of the custom. [M”A 552/10 in name of Levush; Bach; Shlah; M”A 552/12 in name of Darkei Moshe that one is to only eat one cooked dish even during the first meal; brought in M”B 552/22]
 The reason: The reason for eating a larger meal than usual is in order to prevent the fast from causing one damage [due to hunger] as by this fast one is required to stop eating from the night before as is the law by Yom Kippur. [Rama ibid] Alternatively, the reason for this large meal is to commemorate that in the times of the second Temple, T”B was a festive day in which a festive meal was held. By this we express our belief that once more T”B will become a day of rejoicing. [M”A 552/11; Elya Raba 552/11]
 Rama ibid “so appears to me”.
 See M”A 552/11 regarding the alternative reason for increasing in this meal.
 M”B 552/22 in name of Elya Raba; Michaber 552/9 “The Seudas Hamafsekes must be a Seudas Keva” [and not a Seudas Aray]; M”A 552/10 in name of Tur and Ramban “Not like those that their stomach is the stomach of Reshaim as they satiate themselves first and then have a mere snack as Seudas Hamafsekes”; Kaf Hachaim 552/46
 The reason: The reason for this is because the final meal has a number of mourning customs and limitations, and if one will not properly eat the final meal due to being overfull, his final meal is considered to have been eaten before mincha, without following the mourning limitations. [M”B 522/20; Poskim ibid]
 Rama 552/9
 M”B 552/21; See Shaar HaTziyon 552/16
 Michaber 552/12; Vetzaruch Iyun why Admur in the Siddur does not list Mincha of Erev Tishe Beav as days that we omit Tachanun. [Siddur with Haaros Raskin p. 176]
 Shaar HaTziyon 552/16
The reason: As otherwise the second meal is considered a Bracha Sheiyno Tzricha. [ibid]
 Based on Shaar HaTziyon ibid
 Such as if he will be Davening Mincha Gedola and then eating the first meal, and then eat the second meal right before sunset then it is not considered a Bracha Sheiyno Tzricha even though he did not Daven Mincha in between the two meals. [Kaf Hachaim 552/47]
 Kaf Hachaim 552/47
 Michaber 552/9
 Michaber 552/1; Mishneh Taanis 26b
 Michaber 552/2
 M”A 552/2
 Michaber 552/3
 Michaber 552/3; M”A 552/4
 Michaber 552/4
 Rama 552/6
 M”A 552/6
 The reason: As hardboiled eggs is the food of a mourner [Rama ibid]
 Rama 552/1
 M”A 552/1
 M”A ibid in name of Mahril; Aguda; Rashal; Hagahos Maimanis
 M”A ibid in name of Mateh Moshe
 Shaareiy Teshuvah 552/2 in name of Machazik Bracha; Aruch Hashulchan 552/9; Shearim Hametzuyanim Behalacha 123/3; Piskeiy Teshuvos 552/5
The reason: As drinks are not included within the two cooked foods. [Machazik Bracha ibid] Alternatively, it is because it is not considered a cooked dish, being that it is not cooked in a Keli Rishon but rather through Iruiy Keli Rishon. [Shearim ibid based on Ashel Avraham Butchach ibid]
 Zera Emes brought in Machazik Bracha ibid; Shaareiy Teshuvah ibid
 Michaber 552/6; See 552/5-6; It appears that one who will not be satiated with only eating the above, may eat in accordance to the Halachic guidelines mentioned above
 Rama ibid
 Piskeiy Teshuvos 552/8
 Rama 552/6; Kitzur SHU”A 123/3; Nitei Gavriel 51/9
 M”B 552/16
 Michaber 552/7
 Maharash Shaar Aryeh; Birkeiy Yosef 552/8; Shaareiy Teshuvah 552/3 in name of Birkeiy Yosef ibid; Kaf Hachaim 552/39
Other opinions: From the Maharil brought in Beir Heiytiv 559/3 it is implied he held that one may sit directly on the ground. [Shaareiy Teshuvah ibid] Practically, however, one is to be stringent in this matter. [Shaareiy Teshuvah ibid; see P”M 559 A”A for how he explains the Maharil; See also next footnotes from Kaf Hachaim who differentiates between a tiled and earth floor]
 Shaareiy Teshuvah 552/3 in name of Birkeiy Yosef
 Kaf Hachaim ibid suggests that perhaps the above stringency of Kabala is only by an earth floor and thus there is no contradiction between the Maharil and the Maharash however he concludes “When possible one is to be stringent even regarding a tiled floor”.
 Michaber 552/8
 M”B 552/19
 Michaber 553/1; Admur 608/7
 So it seems, that even after one closes his Seudas Hamafsekes he still remains limited in what he can eat.
 M”B 553/2; Admur ibid regarding Erev Yom Kippur however there he differentiates between accepting the day of Yom Kippur in which case the above law applies, and when he simply accepts to no longer eat.
 M”A 553/2 in name of Bach; Gr”a; M”B 553/2; Admur 608/7 regarding the Seudas Hamafsekes on Erev Yom Kippur however there he differentiates between accepting the day of Yom Kippur in which case even acceptance of the heart is valid, and when he simply accepts to no longer eat in which case it is only valid if verbalized.
Other opinions: Some Poskim rule the acceptance is only valid if one verbalized it with his mouth, as words of the heart have no validity. [Rama 553/1]
 Admur 608/7; M”A 553/2; M”B 553/2
 M”B ibid
 M”B ibid
 M”B ibid; see Admur ibid
 Taharas Hashulchan 557; Nitei Gavriel 66/10
 The reason: As this washing is not a washing of pleasure.
 Shaareiy Teshuvah 552/13; Nitei Gavriel 51/21
 Rama 552/6; Kitzur SHU”A 123/3
 Moed Lekol Chaiy 10/29; Nitei Gavriel 51 footnote 21; Hiskashrus; See Ashel Avraham Butchach 552