Just as there is a mitzva to extend the sanctity of Shabbat into Friday, there is a mitzva to extend it into Saturday night. Therefore, one must be careful not to perform any melakha until several minutes after tzeit. After that, according to Torah law one may resume melakha even without making havdala, but the Sages enacted that one may only do melakha after reciting havdala in the berakha of Ata Ĥonen in the Amida or by reciting the phrase “barukh ha-mavdil bein kodesh le-ĥol” (“blessed is the One Who distinguishes between the sacred and the mundane”). Before this recitation, one may not even perform melakhot that are rabbinically prohibited (SA 299:10).
Because of the importance of reciting havdala over a cup of wine, the Sages prohibited eating and drinking after shki’a until one makes havdala over wine. However, one may drink water then, as it is not deemed significant (SA 299:1). Other Aĥaronim maintain one may not even drink water (Kaf Ha-ĥayim 299:6).
Thus, one may only do melakha after making havdala verbally, and one may only eat or drink after making havdala over wine.
Most poskim maintain that when making a zimun at se’uda shlishit over a cup of wine, the leader of the zimun drinks from the wine after Birkat Ha-mazon. Although it is already after tzeit, drinking the wine is deemed a continuation of the meal. Just as one who began se’uda shlishit may continue eating even after shki’a and tzeit, so too one may drink the wine from the zimun (SA 299:4).
Others maintain that since people do not always insist on making a zimun over a cup of wine, the wine is not considered a direct continuation of se’uda shlishit, and therefore one may not drink it before havdala (MA, MB 299:14). Those who follow this opinion save the cup of wine from the zimun until after Ma’ariv, when they make havdala over it. If newlyweds are present at se’uda shlishit, since Sheva Berakhot are recited over the cup of wine, one also makes the berakha over the wine then. The person leading the zimun drinks from it, as do the bride and groom.
Once Shabbat has ended, one may make havdala over wine even before praying Ma’ariv (MA 489:7; MB 18). When he later prays, he should recite Ata Ĥonantanu. The one reciting havdala must remember not to drink a revi’it of wine, though. If he does so, he is considered under its influence and may not pray until the wine wears off (Peninei Halakha: Prayer 5:11).
. If one has no access to either wine or ĥamar medina for havdala, but knows that he will have access by midday the next day, many poskim maintain that he may not eat or drink until he makes havdala over wine on Sunday (Rosh). If he is weak and finds it difficult to fast, he may be lenient and rely on those who maintain that since he does not have wine for havdala, he may eat on Saturday night (SA 296:3; MB 21).. SA 299:4 rules that one should drink from the wine used for Birkat Ha-mazon, even after tzeit on Saturday night. Rema and MB 299:14 state that this is specifically when one usually uses wine for the zimun, following the opinions of Tosafot and Rosh. But for those who follow Rif and Rambam, who maintain that it is not necessary to do a zimun over a kos, it is forbidden to drink from the cup before havdala (see Peninei Halakha: Berakhot 5:13). If one concluded the berakha during bein ha-shmashot, when it is unclear whether it is day or night, he may drink the wine. Some maintain that even if there is no obligation to use wine for a zimun, one who wishes to do so fulfills a mitzva and may therefore drink the wine even before havdala. In practice, there are many who do not drink from the wine used for the zimun at se’uda shlishit, while many Sephardim do. In any event, regarding Sheva Berakhot, the vast majority of poskim maintain that one makes the berakha over the wine and drinks it. See Igrot Moshe OĤ 4:69; Minĥat Yitzĥak 3:113; SSK 59:17; and Yalkut Yosef 291:19.