The Shulchan Aruch 552:10) rules: “If Tisha B’Av falls on Sunday, one may eat meat and drink wine during the seudah mafsekes [last meal before the fast], and one can even bring to his table delicacies that were on the table of Shlomah HaMelech when he was king (i.e. one can eat as much as he wants, and what he wants), however, one must stop before shkia.” The Mishnah Berurah (s.k. 23) adds: “However, one should sit there in a solemn mood and not rejoice, therefore, he shouldn’t eat with friends.” However, the Mishnah Berurah brings that the Bechor Shor argues and maintains that if one normally has friends round for seudas shlishis and he refrains from doing so, it is considered mourning in public, therefore, he may do so this week as well. Either way, everyone agrees that one may eat with family members and make zimmun if there are three adult men eating together.
R' Elyashiv brings an interesting proof to the opinion of the Bechor Shor from a Gemara in Bava Metzia (86b). It’s clear from the Gemara that the seudah [feast] of Avraham Avinu was greater than that of Shlomah HaMelech, so why does the Shulchan Aruch mention, “like Shlomah HaMelech when he was king” and not “like that of Avraham Avinu”? It must be because the Shulchan Aruch is coming to teach us that one may invite guests (cited in Menashim B’Ohel, of R’ Ezriel Auerbach shlita, pg. 148).
Based on the above, one may even bentch with zimmun using a cup of wine, however, one must be careful to drink it before shkia. The Mishnah Berurah (s.k. 24) writes: “It’s correct to notify people that they must finish eating and drinking before shkia, so that people don’t come to accidentally think that they may continue eating and drinking after shkia like on a regular Shabbos”.
R' Moshe Feinstein (Igros Moshe, Orach Chaim 4:112) writes that one may sing zemiros at seudas shelishis, even if he doesn’t normally do so, due to kavod Shabbos (not like the opinion of R’ Sroya Davlitsky zt”l in his sefer Tisha B’Av Shechal B’Yom Rishon, siman 25, who says that even if one normally sings zemiros by seudas shelishis, this Shabbos he should recite them as opposed to singing them).
Since there is an issur to prepare on Shabbos for motzei Shabbos, although one may eat more than normal so that he can fast better, he shouldn’t say out loud to his family members that he is eating more in order that he can fast better the next day (Shemiras Shabbos KeHilchosah 28, 85, and Shu”t Machzeh Eliyohu 1: end of siman 57).
The Kaf HaChaim (553:18) writes that if one is accustomed to learning at his Shabbos table (i.e. Hilchos Shabbos, Chofetz Chaim etc.), he should continue doing so, since stopping to learn after chatzos on erev Tisha B’Av is just a minhag, and some allow it, therefore, one should follow the opinion of those who are lenient.
Washing Mayim Achronim After Shkia
If one got delayed and only ended up reciting birchas hamazon after shkia, and he hasn’t yet washed mayim achronim, even though on this Shabbos after shkia there is an issur for one to wash his hands (see Chayei Adam, Klal 136, 1 and Kitzur Shulchan Aruch 125:1) nonetheless, one may wash his hands during bein hashmoshas [twilight], as even on Tisha B’Av itself one may wash mayim achronim (if he needs to eat) as it is not considered a washing of pleasure (see Shu”t Rivevos Ephraim Vol. 1, siman 384 who cites many poskim who are lenient), therefore, certainly during bein hashmoshas there is room for leniency (R’ Sroya Davlitsky zt”l in his sefer Tisha B’Av Shechal B’Yom Rishon, siman 27).
