77:16 Every Jew, both male and female, is obligated to eat three meals on Shabbos, one at night and two during the day. (If one neglected to eat at night, he must eat three meals by day – Rema 291:1. See Mishnah Brurah 291:5 for how to perform Kiddush under such circumstances.) One must have bread at each meal; one should be very careful to eat bread even at the third meal (colloquially referred to as “shalosh seudos”). Since one washes hands and recites the bracha of al netilas yadayim, he needs to eat at least an egg-sized portion of bread (k’beitzah - 2.2 ounces). (Actually, one should really eat a little more bread than this but if one cannot, then even a k’zayis – 1.1 ounces – will suffice - Mishnah Brurah 291:2.) A person should not overstuff himself at lunch so that he will be able to fulfill his obligation to eat three meals on Shabbos. If one cannot obtain regular bread, he should at least have some cake or another food made from one of the five species of grain over which one would recite the bracha of mezonos. If this is likewise not possible, one should eat food that is normally eaten with bread, like meat or fish. If this is not possible, one should eat cooked fruit (or even uncooked fruit – see OC 291:5). The time for the third meal starts at “mincha gedolah,” which is from the sixth and a half hour of the day. (If one started early but the meal lasted past mincha gedolah, he has fulfilled his obligation – MB 291:7.)
77:17 At each meal, HaMotzi must be recited over two whole loaves. (If one has no whole loaves, he may use partial loaves even as small as a k’zayis – MB 274:2.) One holds the loaves in his hand while reciting the bracha and then cuts one of them (the bottom loaf on Friday night and the top loaf on Shabbos day - OC and Rema 274:1). It is customary to use the knife to mark the place on the loaf where one will cut. The reason is because during the week one must partially cut the bread before reciting HaMotzi as stated in 41:3. On Shabbos this is not possible because one must recite the bracha over whole loaves, so he marks the place so he will know where to cut and not waste too much time looking for the spot. One should place the loaves so that the one to be cut is in front; in this way he does not have to bypass a mitzvah by not cutting the closest loaf. Even if one eats several meals, each meal requires two whole loaves. Similarly, one who makes Kiddush on cake after shul but before lunch should use two whole cakes.