The Shulhan Aruch (Orah Haim 291) emphasizes the importance of Se’uda Shelishit – the third Shabbat meal. The Gemara in Masechet Shabbat comments that those who ensure to eat three meals on Shabbat are spared several calamities, including the war of Gog U’maggog and the suffering that will precede the arrival of Mashiah. The Shulhan Aruch writes that a person "should be exceedingly careful" ("Yeheh Zahir Me’od") with regard to the third meal.
The Hid"a (Rav Haim Yosef David Azulai, 1724-1806), in Mahazik Beracha, suggests an insightful reason for the particular importance of Se’uda Shelishit. The other two meals, he explains, are not evidently eaten for the sake of the Misva, because people in any event normally eat dinner at night and lunch in the early afternoon. Se’uda Shelishit, however, is a meal which people do not normally eat, and thus by eating Se’uda Shelishit, we show that all the Shabbat meals were eaten for the sake of the Misva. For this reason, Se’uda Shelishit is considered especially significant and important.
The Shulhan Aruch writes that one must ensure when eating lunch on Shabbat not to eat so much that he will be unable to eat Se’uda Shelishit. Preferably, one should eat more than a Ka’besa – that is, more than 2 ounces – for Se’uda Shelishit. If one cannot eat more than a Ka’besa, he should eat at least a Ka’besa, or, if he cannot, then a Ke’zayit. If a person cannot eat at all, then he is not required to eat, as the obligation of Se’uda Shelishit is to enjoy a meal, not to experience discomfort as a result of eating.
Women are obligated in Se’uda Shelishit just as men are. Often, men eat Seu’da Shelishit in the synagogue while their wives are home, and the wives are unaware of their requirement to eat Se’uda Shelishit. They must be informed that the Se’uda Shelishit requirement applies equally to men and to women.
The Halachic authorities debate the question as to the preferred time for Se’uda Shelishit, as discussed by the Rama (Rav Moshe Isserles of Cracow, 1525-1572). One opinion is that Se’uda Shelishit should preferably be eaten before Minha, since according to one view it is improper to drink anything between Minha and Arbit. Others maintain that one has the option to eat Se’uda Shelishit either before or after Minha, and neither option is preferred over the other. The Rabbis of Kabbalah held a third view, namely, that one should ensure to eat Se’uda Shelishit specifically after Minha. This is, indeed, the accepted custom. However, if a person will be unable to eat Se’uda Shelishit after Minha, then he may eat it earlier, as long as the earliest time for reciting Minha has already arrived (6.5 Halachic hours into the day). Thus, for example, if a person knows that he will not be able to eat Se’uda Shelishit after Minha, and he had a meal with bread at the synagogue after Shaharit, he may then have in mind that his lunch at home should be considered Se’uda Shelishit, and he fulfills his obligation in this manner. Ideally, however, as mentioned, one should eat Se’uda Shelishit after Minha.
Summary: One should ensure not to eat too much at lunch on Shabbat, so that he will be able to fulfill the requirement of Se’uda Shelishit. One should eat at least a Ke’zayit for Se’uda Shelishit, but preferably more than a Ke’besa (two ounces). Women are obligated to eat Se’uda Shelishit just as men are. Se’uda Shelishit should preferably be eaten after Minha. If this is not possible, it should be eaten no earlier than the earliest time for Minha.
