Gemara

(Beraisa) Question: How many meals must one eat on Shabbos?

Answer #1: He must eat three meals;

Answer #2 (R. Chidka): He must eat four meals.

(R. Yochanan): Both of them expound the same verse:

"Ichluhu ha'Yom Ki Shabbos ha'Yom la'Shem ha'Yom Lo Simtza'uhu ba'Sadeh" - R. Chidka holds that the three times it says "ha'Yom" allude to three meals [during the daytime], not including the Shabbos night meal;

Chachamim hold that they allude to three meals including the Shabbos night meal.

118b (Rav Yehudah bar Shmuel): To fulfill Oneg Shabbos, one eats a dish of beets, big fish and heads of garlic. (These were prestigious foods.)

(R. Chiya bar Ashi): Even a small matter for the honor of Shabbos fulfills Oneg.

(Rav Papa): This small matter is Kisa d'Harsena (fish fried in its own oil with flour).

Pesachim 105a (Beraisa): Honor of the day meal has precedence over honor of the night meal. If one has only one cup of wine, he says Kidush ha'Yom over it, for this has precedence over honor of the day meal.

Gitin 38b (R. Chiya bar Aba): There were two families in Yerushalayim that were uprooted. One fixed meals on Erev Shabbos, and the other, on Shabbos day [at the time of the lecture in the Beis Midrash].

Rishonim

Rambam (Hilchos Shabbos 29:4): The primary Kidush is at night.

Rosh (10:13): Honor of the day meal has precedence over honor of the night meal. Kidush at night has precedence over both of them.

Tosfos (Pesachim 113a DH d'Meshayer): Why does he save the leftover wine from Kidush ha'Yom for Havdalah [and not drink it during the day meal]? Only Kidush ha'Yom has precedence over Kevod Yom, but Havdalah does not have precedence over it! The Ri explained that Kidush Yom is called Kivud Yom. The Yerushalmi says "Kivud Yom is Borei Peri ha'Gafen."

Rashi (38b DH b'Erev): One family fixed meals on Shabbos night. This is not honorable to Shabbos, for we hold that honor of the day is greater. They intended to avoid Bitul Beis Midrash. (If the day meal were longer, they would miss the lecture in the Beis Midrash.) Some say that they fixed a meal on Erev Shabbos. They did so on every day, for they were wealthy. It is forbidden on Erev Shabbos, for then they enter Shabbos satiated, without appetite.

Yam Shel Shlomo (Gitin 4:51): One should be concerned for both Perushim, for both are good, especially since families were uprooted due to this. I am distressed by Shomrei Shabbos who are not careful about this. Not only do they equate the morning and evening meals. They even add more courses to the night meal than to the day meal. They have good fish, which is the primary honor of the meal, at night. One must show a preference to the day. Once I recognized what is my proper path, I fenced myself not to eat fish on Friday night, only in the morning. This way, even if there are many delicacies at night, it does not equal the morning meal, which has fish.

Sha'arei Teshuvah (OC 271:3): If a certain food is dear to a person, he should leave it for the day meal.

Poskim

Shulchan Aruch (OC 271:3): If one cannot afford to buy wine for Kidush and nice food to honor the night and day meals, wine for the night Kidush has precedence. Even though a Beraisa says that the day meal has precedence over the night, this is only regarding other food. If one has only one cup for Kidush, the honor of the night has precedence.

Beis Yosef (DH v'Im): The Rosh's text says that Kidush at night has precedence over honor of the day and night meals. Our text says that Kidush ha'Yom has precedence over honor of the day and night. The Tur explains that Kidush ha'Yom refers to the night Kidush, like the Mishnah (Berachos 51a) says "first he blesses on the wine, and then on the day." The night Kidush has precedence because it is mid'Oraisa, but the day Kidush is only mid'Rabanan. Tosfos brings from the Yerushalmi that Kivud Yom is Borei Peri ha'Gafen. I.e. wine for the night Kidush has precedence over wine for the day Kidush. This is like the Tur. Also Shibolei ha'Leket says that Kevod Yom has precedence over Kevod Laylah for other needs of the meal, but if one has only one cup for Kidush, Kevod Laylah has precedence. A Beraisa expounds first Kidush at night, and then Kidush of the day, from "Es" [which always connotes what is secondary].

Magen Avraham (3): If one cannot afford bread and wine for Kidush, bread has precedence, for Kidush is only b'Makom Seudah. Also, one is obligated to eat bread. If one has wine for the night Kidush, perhaps foods to honor the day meal override wine for the day Kidush. The Ran says that the day Kidush was enacted because honor of the day is greater than that of the night! However, perhaps it suffices to have bread, and to use what remains for wine [for Kidush ha'Yom - Mishnah Berurah 7].

Mishnah Berurah (7): Honoring the meal is derived from the Nevi'im, "v'Karasa la'Shabbos Oneg." The source for night Kidush is mid'Oraisa (even though mid'Oraisa one fulfills Kidush verbally without wine). Bread for the night and day meals is obligatory, therefore it has precedence over wine; night Kidush can be said over bread.

Bi'ur Halachah (DH Mutav): If one cannot afford wine for Kidush and food to honor the meals, and he prefers bread to wine, he should buy food and make Kidush on bread to fulfill both Mitzvos. This is not only according to the Beis Yosef, who holds (Siman 272) that if one prefers bread to wine, he may always say Kidush on bread. Surely, he should do so here. Even the Rema, who holds there that when there is wine in the city, we do not say Kidush on bread, admits here. The Rema is concerned for R. Tam's opinion [that one may not say Kidush on bread], but he does not rule like him. This is clear from Darchei Moshe ha'Aruch. There [when he prefers bread], no calamity will result if he says Kidush on wine. Here, he would neglect Oneg Shabbos [if he does not buy enough bread, so the Rema agrees that] we need not be concerned for R. Tam's opinion, and he is Mekadesh on bread. I brought from the Magen Avraham that if one lacks money for wine for the morning Kidush and needs of the morning meal, he buys wine for Kidush, and eats just bread, and fulfills Kevod Yom through this. Seemingly, Oneg is a Mitzvas Aseh from Divrei Torah Kabalah (Nevi'im), and one does not fulfill this through tasteless bread [without accompaniment], like we find in 118b! Where do we find Kevod Yom? Perhaps the Magen Avraham means that one eats bread with Kisa d'Harsena, and uses the rest of his money for wine.

Magen Avraham (4): The Zohar teaches that one should set the table at night with bread and other foods. R. Yitzchak says, also during the day.

Kaf ha'Chayim (14-15): Enough bread to be sated has precedence over wine. One need not afflict himself. Lechem Mishnah is not so obligatory, so wine for Kidush has precedence over it.

Mishnah Berurah (8): Even though one may say Kidush on bread at night, but not during the day, the night has precedence, for when there is wine in the city, we do not say Kidush on bread.

Mishnah Berurah (9): If one has a limited amount of delicacies he should leave them for the day meal.

Kaf ha'Chayim (16): Maharshal would have fish only by day to honor the day. The Ari Zal says to have fish at night, so one should refrain from a different delicacy at night.