Eating before Havdala:
It is forbidden to eat, drink or even taste any amount of food or beverage prior to saying or hearing Havdala over wine or other valid beverages. This however is with exception to water which is allowed to be drunk prior to Havdala. [Nevertheless our custom is to avoid drinking even water prior to Havdala.]
May one eat after sunset before nightfall? It is forbidden to eat or drink anything other than water after sunset until one says Havdala. [Furthermore, the custom is to avoid drinking even water from after sunset until one says Havdala.] If one began eating or drinking after sunset he must stop as soon as he remembers even if he had begun eating a meal with bread after this time. Nevertheless those which are accustomed to begin large meals after sunset [but before nightfall] are not to be protested as they have upon whom to rely. According to all once nightfall has arrived it is forbidden to eat or drink, and if one began eating or drinking at this time he must stop as soon as he remembers.
The third meal: [The above restrictions apply even regarding the third meal of Shabbos [i.e. Shalosh Seudos]. Hence one must begin his third meal prior to sunset. Once sunset has arrived he may no longer eat the third meal. However there are Poskim which are lenient to allow the third meal to be eaten even past sunset, until nightfall, if one did not eat it before hand. Many are accustomed to be lenient like this opinion. In such a case it is disputed amongst Poskim whether one is to say Ritzei in Birchas Hamazon. According to all once nightfall arrives it is forbidden to begin eating the third meal, even if he is still within the time of Tosefes Shabbos.]
If one already said the blessing over food and then remembered the prohibition: In the event that one said a blessing over food after sunset, or even after nightfall and then prior to eating the food remembered the above prohibition, he is to taste the food and then say Havdala. [If however one merely washed for bread and did not yet say Hamotzi, and then remembered the prohibition, he is to say Havdala right away and then say Hamotzi.]
If one began eating bread before sunset: If one began eating a meal [that consists of bread] prior to sunset he may continue to eat and drink without limit even after nightfall. [This applies even if he has not yet eaten a Kezayis worth of bread prior to sunset, but has said Hamotzi and thus started his meal prior to sunset.] If however one did not begin an actual meal [that consists of bread] prior to sunset but was merely drinking [or eating foods other than bread] he must stop eating and drinking as soon as sunset arrives.
If during the meal, after sunset or nightfall, the group of people eating decided to say Havdala, they may still change their mind and continue eating. [If however they decided to finish the meal and recite Birchas Hamazon they may not continue eating. Furthermore if after sunset an individual decides in his mind to finish his meal, he may no longer continue eating.]
Saying Havdala in middle of the meal, prior to Bentching:  In the event that one decides to say Havdala in middle of the meal, prior to Bentching, he is to omit the blessing of Hagafen from Havdala if he drank wine during, or directly before, the meal. Likewise if he is saying Havdala over other beverages the blessing of Shehakol is omitted. Nevertheless it is proper to initially avoid saying Havdala prior to Birchas Hamazon in such a case, as explained above in Halacha 1-See there! [If he said Havdala during the meal and desires to continue eating after Havdala, there is no need to say Hamotzi over the foods and he may therefore continue eating.]
If one Davened Maariv in middle of the meal: In the event that one Davened Maariv [or said Baruch Hamavdil] in middle of the meal, he is forbidden to continue eating or drinking until he says Havdala, even if he washed for bread before sunset. This applies even if one Davened Maariv early, prior to dark. [In such a case one is to omit Ritzei from Bentching. Due to this initially one is to avoid Davening Maariv or saying Havdala, or Baruch Hamavdil until after reciting Birchas Hamazon in order for him to be able to recite Ritzei. However there are Poskim which allow doing so even initially. If one answered for Barchu of Maariv prior to saying Birchas Hamazon, he may nevertheless recite Ritzei in Bentching if he recites it prior to Davening Maariv.]
Drinking the Kos Shel Bracha after Birchas Hamazon: One may not drink from the cup of wine used for Kos Shel Bracha during Birchas Hamazon until after Havdala. Thus after Birchas Hamazon one is to leave the Kos Shel Bracha on the table, and not say a blessing over it. The custom is to then Daven Maariv and after Maariv to say Havdala. Unlike the ruling in the Shulchan Aruch, the custom today is to say Havdala over this same cup of wine, as opposed to using another cup of wine for Havdala. [It is best for the person who led the Zimun, or at least participated in Birchas Hamazon, to say Havdala over this cup of wine. In the event that no one will be saying Havdala over the cup of wine, it is nevertheless proper to say Birchas Hamazon over a Kos Shel Bracha.]
It is forbidden to eat or drink prior to Havdala. The custom is to avoid even drinking water. The prohibition begins starting from sunset unless one is in the midst of a meal with bread in which case he may continue eating and drinking even past nightfall, until he recites Birchas Hamazon or Davens Maariv or recites Baruch Hamavdil. One must however begin this meal prior to sunset.
If one said Havdala early, before sunset, may he eat after sunset?
From the letter of the law may one drink water based soft drinks before Havdala?
It is questionable whether soft drinks that are water based are permitted to be drunk before Havdala just as is the law regarding water itself. [Practically the custom is to avoid drinking even water prior to Havdala, and thus certainly water based soft drinks are to be avoided.]
May women eat or drink prior to Havdala?
May children eat prior to Havdala?
Yes. It is forbidden to prevent them from eating if they need to eat.
If one began eating a meal before sunset may he eat fruits, pastries and the like after sunset even though they will require a new blessing?
Yes. Since one is in middle of a meal he may continue to eat all foods that are exempt with his Birchas Hamazon, such as fruits and other desserts.
If one began eating Mezonos before sunset may he continue to do so after sunset?
Some Poskim rule it is forbidden to continue eating past sunset if he did not start a meal which consists of bread beforehand, even if he ate Mezonos. Others rule if he ate Mezonos before sunset he may continue eating it after sunset. Others rule if he intends to fulfill his obligation of the third meal with the Mezonos he is eating then he may continue eating it after sunset.
If one is reciting Birchas Hamazon after sunset but before nightfall, may he drink the Kos Shel Bracha?
Some Poskim rule one may drink from the Kos Shel Bracha even if he is not always particular to have a Kos Shel Bracha while reciting Birchas Hamazon. However seemingly according to Admur it is proper to be stringent to avoid drinking the Kos Shel Bracha in such a case, although those that are lenient have upon whom to rely.
If one made Sheva Brachos for the third meal, what is one to do with the Kos Shel Bracha?
Many Poskim rule one is to say Sheva Brachos over the wine and say the concluding blessing of Hagafen immediately after Birchas Hamazon, before Havdala. In such a case some say the person saying Birchas Hamazon is to drink from the cup. Regarding how much he is to drink some Poskim rule he is to drink a full Revius of the cup. Others rule he is to only taste a small amount. However some Poskim say only the Chasan and Kala are to drink from the cup and not the person who said Birchas Hamazon. For this reason some are accustomed to have the Chasan say Birchas Hamazon. Some only give the Kala to drink from the cup and not the Chasan.
Some Poskim argue on all of the above and rule one is not to say a blessing over the cup of wine until after Havdala.
To note that some have the custom not to make Sheva Brachos at all by the third meal.
 This applies even if one recited Havdala within Maariv Shemoneh Esrei. [Ibid]
The Reason: The reason for this prohibition is because one is supposed to say Havdala close to the leave of Shabbos without delay. Thus although in general the Sages only forbade a meal from being eaten prior to fulfilling a Mitzvah while snacking prior to the Mitzvah was allowed, nevertheless here even merely snacking or tasting food was forbidden as Havdala is to be said close to the leave of Shabbos and thus even snacking will cause one to delay saying it within this time. [271/9; Kuntrus Achron 299/2]
Severity of eating prior to Havdala: One who eats before Havdala is liable for death through the illness of Askara. [Pesachim 105a]
 This ruling of Admur follows the ruling of the Michaber 299/1
Other Opinions: However the Geonim, and Midrash Talpiyos rule even water is forbidden to drink, and so is the ruling of the Arizal. [Kaf Hachaim 299/6; Ketzos Hashulchan 94 footnote 1; See Maggid Meisharim Vayeishev] and so is the Chabad custom as recorded in Hayom Yom [brought next].
 Sefer Haminhagim p. 68; Hayom Yom 3rd Iyar and so brings Ketzos Hashulchan 94 footnote 1 based on Arizal.
 299/2; See Kuntrus Achron 271/3
Background of this ruling in Admur:
It is disputed in Rishonim [Raza/Rosh/Rambam/Rif] and Poskim [Bach/Magen Avraham 1; Taz 1] if the prohibition of eating and drinking before Havdala begins from nightfall or from Bein Hashmashos, which is after sunset [according to our custom]. Admur [ibid] records this dispute in regards to eating during Bein Hashmashos. His first [and main/stam] opinion is that it is forbidden. The second opinion rules it is permitted until nightfall. Admur in his final ruling rules that the main opinion follows the stringent opinion, and one is thus to be stringent, although those which are lenient are not to be protested.
Other Opinions: This stringent ruling of Admur is based on the ruling of the Bach and Magen Avraham 299/1. However the Taz 299/1 rules that one may be lenient to eat and drink until nightfall and so he learns is also the opinion of the Michaber/Rama.
The Mishneh Berura [299/1] rules like the Magen Avraham that the prohibition begins from after sunset however he then concludes that one may be lenient if he is hungry or thirsty until 30 minutes prior to nightfall [in countries that there is more than 30 minutes between sunset and nightfall]. The basis of this ruling [as learns the Ketzos Hashulchan 94 footnote 6] is that one may be lenient like the opinion of Rabbeinu Tam regarding Bein Hashmashos in this situation. The Ketzos Hashulchan [94 footnote 6] however question this ruling concluding that one who does not follow Rabbeinu Tam for the time he lets out Shabbos is not to be lenient in this at all.
The Ashel Avraham Butchather 299 rules that one may eat even a meal after sunset [even not Shalosh Seudos] so long as there is someone who is not eating who will remind them to do Havdala.
 As once sunset has passed it is Bein Hashmashos which is questionably considered night time, and at night it is forbidden to eat or drink prior to Havdala. [ibid]
Prior to sunset it is permitted to begin the meal even if it is within 30 minutes to nightfall. This does not pose a prohibition of eating a meal within 30 minutes before Maariv. [See Ketzos Hashulchan 94 footnote 6]
 Hayom Yom 3rd of Iyar and so brings Ketzos Hashulchan 94 footnote 1 based on Arizal.
 However if they begin after nightfall then they are certainly to be protested.
 So explains also Biur Halacha 299 “Mishetechshach” that those which are accustomed to begin large wedding meals after sunset have upon whom to rely. His reasoning there is because it is a Seudas Mitzvah and perhaps by a Rabbinical prohibition we are not stringent to force one to avoid fulfilling a Seudas Mitzvah and one thus can rely on the lenient opinions mentioned above. In the Kuntrus Achron [271/3] Admur explains that since one is unable to make Havdala until after nightfall therefore there is more room to be lenient, as opposed to Kiddush which can be made by Bein Hashmashos.
Furthermore, the Biur Halacha is lenient to allow one who is thirsty or hungry to eat or drink up to 30 minutes before Tzeis Hakochavim, even if it is after sunset. His reasoning is because one may rely on the opinion of Rabbeinu Tam regarding this matter that Bein Hashmashos does not begin until this time. The Ketzos Hashulchan [94 footnote 6] however questions this ruling of the M”B based on that today we do not follow Rabbeinu Tam regarding Melacha. [See Az Nidbaru 13/22; Piskeiy Teshuvos 299/3]
 Pashut from Admur, as no differentiation was made. See Piskeiy Teshuvos 299 footnote 31 that Chazon Ish did not allow people to wash for the third meal after sunset.
 M”B 299/1; Ashel Avraham Butchacher ibid regarding if there is someone which is not eating to remind him to say Havdala.
 As the eating of the third meal is a Rabbinical command and hence in such a case one may rely on the opinion of the Razah/Taz which allow food to be eaten until nightfall. Furthermore it was already stated above that the custom is to be lenient to begin large meals of weddings after sunset. Furthermore one can say that a Rabbinical command to eat the third meal differs the Rabbinical prohibition of eating after sunset. [M”B ibid; Shaareiy Tziyon 2; Biur Halacha “Mishetechshach”]
 Piskeiy Teshuvos 299/3
 Some Poskim [Ketzos Hashulchan 92 footnote 8] rule that if one did not eat a Kezayis of bread before sunset he may not say Ritzei in Birchas Hamazon. Others [Ashel Avraham Butchacher 188/19] rule even if he ate a Kezayis of bread after sunset Ritzei is to be recited in Birchas Hamazon.
 Piskeiy Teshuvos 299/3
 One is to swallow the food and not merely taste it and spit it out. [See Admur 167/9; Seder 9/1; Kaf Hachaim 89/6] This is unlike the ruling of Mur Uketzia 210; Shut Kol Gadol 72 that rule so long as one had intent to eat it, if he spits it out it is not a blessing in vain. [See Piskeiy Teshuvos 210/9 Vetzaruch Iyun on his omitting the ruling of Admur brought above.]
 The reason he is to taste the food is because otherwise the blessing will be in vain. It does not help to immediately say Havdala and then taste the food after Havdala as this is considered an interval, between the eating and the blessing and a new blessing would be required hence causing the first blessing to be in vain. [ibid]
 So is implied from Admur 166/1 in end; Kitzur Halachos 299 footnote 6; Machatzis Hashekel 299/2
 However eating other foods over which one does not say Hamotzi and Birchas Hamazon is not defined as eating a meal of which its law will be discussed next. [Ketzos Hashulchan 94 footnote 3] Thus even if he ate Mezonos he must stop by sunset. [Aruch Hashulchan 299/5]
Other Opinions: There are Poskim which rule that if one began eating Mezonos prior to sunset for the sake of fulfilling his third meal then he may continue eating after sunset. [See Q&A]
 As since he has begun eating his meal at a permitted time he is allowed to continue it past the permitted time, for as long as he desires. [ibid]
This follows the custom brought in Rama 299/1 and the first opinion in Michaber ibid. However the second opinion in Michaber rules one is to stop his meal by night and say Havdala and only then continue eating.
 Ketzos Hashulchan 94 footnote 4 in name of Peri Megadim.
 Ketzos Hashulchan 94 footnote 3; See Q&A
 The reason for this is because Havdala is to be said close to the leave of Shabbos and thus there is worry that even snacking can cause one to delay saying it within this time. [Kuntrus Achron 299/2]
 Ketzos Hashulchan 94/5
Even though in general we rule one may continue eating the meal without a blessing if he later changes his mind, regarding the third meal on Shabbos one is to be stringent due to the severity of eating before Havdala. [ibid]
 299/4 Kuntrus Achron 3
 Ketzos Hashulchan 94 footnote 7 and 6 based on the ruling of Admur that once one has said Baruch Hamavdil he may not say Ritzei in Birchas Hamazon.
Other Opinions: Elya Raba 299 rules one may continue eating after saying Baruch Hamavdil within a meal. So rules also M”B in Shaareiy Tziyon 299/8
 Based on glosses of Chasam Sofer in end of 263
 See Piskeiy Teshuvos 299/4 footnote 41
 Beir Moshe 1/5
 Admur 299/6 records a distinction in this matter:
If one is always careful to have a Kos Shel Bracha when he says the grace after meals, then he is to drink from the wine after he finishes Birchas Hamazon, prior to saying Havdala. The reason for this is because according to the opinion which requires Kos Shel Bracha by Bentching, the cup of wine is part of the meal, and the same way one may continue eating a meal past night if he began prior to sunset, so too he may drink this wine.
However one which at times says the grace after meals without a [cup of wine], being that he relies on the opinions which say Birchas Hamazon does not require a cup of wine, it is forbidden for him to drink prior to Havdala from the cup of wine used to say Birchas Hamazon.
Rather he should immediately after Birchas Hamazon say Havdala on another cup of wine, if available, and have the first cup included in its blessing of Hagafen, and then immediately drink it after drinking the cup of Havdala. [ibid] [Practically today we are no longer accustomed to always have a Kos Shel Bracha when saying the grace after meals. Therefore we must say Havdala prior to drinking the cup.]
Other Opinions: The above distinction in Admur is taken from the Magen Avraham 299/7. However the Elya Raba 299/6 rules one may always drink the Kos Shel Bracha before Havdala even if he is not always particular to say Birchas Hamazon over a cup. The Kaf Hachaim [299/20] rules when one is saying a Zimun with three people he may drink the cup prior to Havdala, immediately after Birchas Hamazon.
 Ketzos Hashulchan 94 footnote 9; Tehila Ledavid 299/1; Kitzur Halachos p. 118; Biur Halacha 299 “Mivareich”
Ruling of Admur in Shulchan Aruch 299/7: From the letter of the law, immediately after Birchas Hamazon one is to say Havdala in order not to delay between Birchas Hamazon and the Kos Shel Bracha. The reason for this is because one is required to drink the Kos Shel Bracha immediately after Birchas Hamazon, according to those which rule the cup is an obligation. (Furthermore, even according to those which rule Birchas Hamazon does not require a Kos they nevertheless agree that it is a Mitzvahs Min Hamuvchar to do so, and hence he is to drink it as close as possible to Birchas Hamazon.) [299/7]
Explanation of custom: Tehila Ledavid ibid explains the above ruling of Admur was referring to a person who has already Davened Maariv, in which case we tell him to say Havdala immediately after Birchas Hamazon. If one however has not Davened Maariv he is to first Daven. The reason for this perhaps is because it is better to say Havdala within Shemoneh Esrei before one says Havdala over wine. [Kitzur Halachos ibid]
 Ketzos Hashulchan 94 footnote 9; Tehila Ledavid 299/1; Kitzur Halachos p. 118;
Ruling of Shulchan Aruch 299/7: From the letter of the law, immediately after Birchas Hamazon one is to say Havdala on another cup of wine, if available, and have the first cup included in its blessing of Hagafen, and then immediately drink it after drinking the cup of Havdala. The reason for this is because Havdala and Kos of Birchas Hamazon are two separate Kedushos, and hence require two separate cups of wine due to the rule “we do not fulfill Mitzvos in bundles”. (This applies even according to the opinion which rules Birchas Hamazon does not require a Kos, as nevertheless it is a Mitzvah Min Hamuvchar to do so, and hence one should not use the same Kos for both Havdala and Birchas Hamazon.)
However if one does not have another cup of wine readily available then he should say Havdala on the same cup he used for Birchas Hamazon. He should not delay waiting for more wine as one is required to drink the Kos Shel Bracha immediately after Birchas Hamazon, according to those which rule the cup is an obligation, (and even according to those which hold it is not an obligation it is nevertheless a Mitzvah Min Hamuvchar to do so). [299/7]
Other Opinions: Ashel Avraham Butchacher 299 rules one may always say Havdala over the same cup of wine used for Birchas Hamazon, as since the wine for Birchas Hamazon is no longer an obligation the concept of not doing Mitzvos in bundles does not apply.
Explanation of our custom today: Tehila Ledavid ibid explains the above Halacha in Admur was referring to a person who has already Davened Maariv, in which case we tell him to say Havdala immediately after Birchas Hamazon. If one however has not Davened Maariv he is to first Daven. In such a case there is no longer a problem in using the same cup for Havdala, as there has been an interval between Birchas Hamazon and the blessing over the Kos Shel Bracha. This is based on Admur 299 Kuntrus Achron 4 which rules the rule of “Not to perform Mitzvos in bundle” only applies if there is not a large interval between them.
Others answer that our custom is based on the Rambam which allows and encourages using the same cup for Havdala, and we hence did not accept this current ruling of the Shulchan Aruch. [Ketzos Hashulchan ibid] See Kitzur Halachos ibid
 Ketzos Hashulchan 94 footnote 9
As the person who said Birchas Hamazon is to say the blessing over the wine afterwards. Perhaps however today that everyone says Birchas Hamazon quietly to themselves anyone who participated in the meal can say this blessing. [ibid]
 Sheivet Haleivi 8/242
 Peri Megadim 299 A”A 1; Kaf Hachaim 299/4
 Piskeiy Teshuvos 299/1 footnote 6
 As they are obligated in Havdala.
 269/3 regarding Kiddush
 Piskeiy Teshuvos 299/2
 Ketzos Hashulchan 94 footnote 3; Aruch Hashulchan 299/5
 Oar Letziyon 2/22-8
 Sheivet Haleivi 8/36
 Chayeh Adam 18/20; M”B 299/14
 As in such a case they allow to rely on the Taz/Rosh which rule one may even begin a meal at Bein Hashmashos. However according to the M”A it would remain forbidden to drink the Kos Shel Bracha in such a case even if it is still before nightfall. [Chayeh Adam ibid]
 This follows the general ruling of Admur 299/2 regarding eating by Bein Hashmashos, as Admur rules like the M”A although concludes that those which are lenient like the Taz are not to be protested.
 Ashel Avraham Butchacher 299; Minchas Yitzchak 3/113; Beir Moshe 4/35-36; Taamei Haminhagim Ishus; Tosefus Chaim 8/14 on Chayeh Adam in name of Achronim
 Lekutei Mahrich; Beir Moshe 4/35-36
 Beir Moshe ibid
 Zichron Yehuda 87, brought in Minchas Yitzchak ibid
 Igros Moshe 4/69
 See Minchas Yitzchak ibid; Piskeiy Teshuvos 299/8
 Hagahos Chochmas Shlomo 299
 See Piskeiy Teshuvos 299 footnote 74