The Shulhan Aruch (Orah Haim 291:1) rules that one must eat three meals over the course of Shabbat. This follows the majority view recorded in the Gemara (Shabbat 117), as opposed to the position of Rav Hidka, who held that one must conduct three meals on Shabbat day in addition to the meal eaten the night before. The Sages afforded great importance to this Misva of eating three meals on Shabbat, and established that one who observes this Halacha is guaranteed protection from the hardships that will precede the arrival of Mashiah, judgment in Gehinnom, and the devastating battle of Gog U'maggog.
One must make an effort to begin Se'uda Shelishit, the third Shabbat meal, before sundown. If, for whatever reason, one did not begin Se'uda Shelishit before sundown, then he may still do so within 13.5 minutes after sundown, the period known in Halacha as "Ben Ha'shemashot." This is the ruling of Hacham Ovadia Yosef, in his work Halichot Olam (vol. 3).
Generally speaking, Halacha forbids eating after sundown on Shabbat until one recites Havdala. (The exception to this rule is water, which may be drunk during this period.) Nevertheless, if one began an established meal before sundown, it is permissible for him to continue eating even after sundown. Thus, for example, if one washed his hands and ate bread for Se'uda Shelishit several minutes before sundown, he may continue eating and drinking even after sundown. Since he began a formal, established meal prior to sunset, he may continue the meal even after sunset.
If, however, a person began eating not in the context of an established meal, then he must stop eating once the sun sets. For example, if a person is unable, for whatever reason, to eat bread for Se'uda Shelishit, such as if he feels too full to eat bread, and he instead wishes to eat some fruit, he must discontinue eating at sundown. Partaking of fruit does not constitute an "established meal" with regard to this Halacha, and thus in such a case eating is not permitted after sunset.
What is less clear is whether one may continue eating after sundown if he began eating cake or other "Mezonot" foods before sundown. Does the consumption of such foods qualify as an "established meal" which allows one to continue eating after the sun sets, or does Halacha define an "established meal" as specifically a meal consisting of bread?
According to all authorities, if a person partakes of 8 oz. or more of "Mezonot" food, he is considered as establishing a formal meal and may therefore continue eating after sundown. After all, one who eats this quantity of cake and the like must wash his hands and recite "Ha'mosi" and Birkat Ha'mazon. Undoubtedly, then, eating this amount qualifies as an established meal. The authorities disagree, however, with regard to one who eats lesser amounts of "Mezonot" foods. Hacham Bension Abba Shaul (Israel, 1923-1998), in his work Or Le'siyon (vol. 2), argued that since "Mezonot" food has the potential to become an established meal – if one eats 8 oz. of such food – even lesser amounts suffice to allow one to continue eating past sundown. (Hacham Ovadia Yosef makes no comment on this ruling in his critique of Or Le'siyon, perhaps suggesting that he accepts this position.) By contrast, Rabbi Moshe Halevi (Israel, 1961-2001), in his work Menuhat Ahava (vol. 1), held that if one eats less than 8 oz. of "Mezonot" food, he has not partaken of an established meal and must therefore discontinue his eating once the sun sets.
As for the final Halacha, it is certainly preferable to eat bread with Se'uda Shelishit, which many authorities require in any event and through which one avoids this Halachic debate. If, however, one cannot partake of bread and he began eating cake and the like before sundown, he should consult with his Rabbi for guidance, given the differing views that exist in this regard.
Summary: Halacha requires conducting three meals over the course of Shabbat. The third meal should begin before sundown, though one who could not begin this meal before sundown may do so within 13.5 minutes after sundown. It is proper to conduct all three meals over bread. If one does not eat bread for the third meal, he must discontinue eating once the sun sets, unless he had begun eating cake or other "Mezonot" food, in which case some authorities allow him to continue eating after sundown. In such a case, one should consult with his Rabbi for guidance, given the different views that exist among the Halachic authorities in this regard.
