There are two mitzvot pertaining to the Shabbat meal. One is oneg, the mitzva to delight in Shabbat, as it is written: “Call Shabbat ‘delight’ (oneg)” (Yeshayahu 58:13). Oneg is fulfilled primarily through the meals, but snacks and a Shabbat nap are parts of it as well. The second mitzva is to partake of three meals. The Sages saw this hinted at in various verses (Shabbat 117b).
The first meal is held on Friday night, the second on Shabbat morning before midday, and the third on Shabbat afternoon from half a seasonal hour after midday until shki’a. If one eats the third meal prior to this, he has not fulfilled his obligation (SA 291:2). One who did not eat dinner on Friday night should eat three meals on Shabbat day. If one was unable to eat the second meal before midday, he should eat two meals afterward, for some maintain that the timing of the meals is not critical, and be-di’avad one may rely upon them (Behag; Rema 291:1).
Bread is the staple of the meal because it is the most important food. It is a mitzva to prepare other good foods that people delight in. In the time of the Sages, people enjoyed a dish made of spinach, large fish, and heads of garlic, so it was a mitzva to prepare these for Shabbat (Shabbat 118b; MB 242:1). Since most people enjoy meat, wine, and delicacies (meaning tasty fruits), poskim write that we should have plenty of them (SA 250:2). One who does not enjoy meat and wine should prepare foods he does enjoy for Shabbat.
The Aĥaronim write, based on Kabbala, that there is a mitzva to eat fish at each of the three meals. Several reasons are given for this: fish symbolize blessing, they hint at deep matters since they are creatures of the deep, and ayin ha-ra (the evil eye) has no power over them. However, one who does not enjoy fish is not required to eat it (MA 242:1).
Even though eating sparingly is generally a positive character trait, on Shabbat it is a mitzva to eat heartily. It is not considered gluttonous since it is for a mitzva (Shabbat 117b; SA 274:2; MB 6). However, one should not overeat, because overeating leads to exhaustion and depression. As for those who stuff themselves, fill up, become tired, fall asleep, and do not study Torah, they do not get any credit for the mitzva. They are not making Shabbat enjoyable, they are only pleasing their gullets (Shlah, Masekhet Shabbat, Ner Mitzva §37; see above 5:3).
One may not fast on Shabbat, even for just an hour. Even one who does not intend to fast but in fact has not eaten anything by midday on Shabbat morning has transgressed this prohibition (SA and Rema 288:1). He also is obviously not eating the second meal at its ideal time.
One who is ill and has no appetite need not eat very much, since the eating is meant to be pleasurable. One who does not enjoy eating need not eat much but should try to eat a little more than a keveitza (egg’s bulk) of bread. If even this is difficult for him, he should eat at least a kezayit. If even this amount pains him, he should not eat at all (SA 288:2; 291:1).
. According to Sefer Ha-ĥinukh §297, the mitzva of oneg Shabbat is rabbinic since its source is Yeshayahu 58:13, and mitzvot derived from the Prophets are similar in status to rabbinic mitzvot. However, according to Rambam it is a Torah law derived from “But on the seventh day there shall be a Shabbat of complete rest, a sacred occasion (mikra kodesh)” (Vayikra 23:3). Implied by the term “mikra kodesh” is honoring Shabbat with good food and clean clothes (MB 242:1). The question of whether fasting on Shabbat is a Torah prohibition or a rabbinic one hinges on this disagreement. See BHL 288:1.The mitzva of eating three meals, according to almost all poskim, is rabbinic. According to Shabbat 117b it is hinted at in the verse in Shemot 16:25 where the word “day” appears three times: “Then Moses said, ‘Eat it today, for today is a Shabbat of the Lord; you will not find it today on the plain.’” However, there is an opinion that this is an actual derivation from the verse and thus the mitzva would be of Torah origin (Yere’im §92; Levush). AHS 291:1 states that even if this mitzva is not literally from the Torah, it must have been instituted by Moshe as a Sinaitic tradition. See SSK ch. 54 n. 109.
. Although as long as one eats a kezayit of mezonot at kiddush (or a revi’it of wine according to some), he fulfills the requirement of making kiddush in the place of a meal, this is because kiddush need not be made where one of the three Shabbat meals will be eaten. Rather the obligation is that it be made where one experiences oneg. Eating a kezayit is enough to meet this requirement. But in order to have one’s eating be considered one of the three required Shabbat meals, he must eat a set meal. The smallest amount for this is more than a keveitza, although be-di’avad a kezayit will do (MB 291:2; SSK ch. 54 n. 4; see Menuĥat Ahava 1:8:2).