Gemara

(Beraisa - R. Eliezer): On Yom Tov a person must eat and drink, or learn;

R. Yehoshua says, one should divide the day - half to eat and drink, and half to learn.

(R. Yochanan): Both of them learn from the same verses:

Contradiction: It says "Atzeres la'Shem Elokecha," and it says "Atzeres Tihyeh Lachem"!

Resolution #1 (R. Eliezer): It must be all for Hash-m, or all for you.

Resolution #2 (R. Yehoshua): One must divide the day - half for Hash-m, and half for you.

(R. Elazar): All agree that on Shavu'os, part of the day must be for you.

(Rabah): All agree that on Shabbos, part of the day must be for you.

We learn from "v'Karasa la'Shabos Oneg."

Mar brei d'Ravina used to fast every day, except for Shavu'os, Purim and Erev Yom Kipur.

Rishonim

Rif and Rosh (Beitzah 8a and 2:2): All agree that Shabbos must be also for you.

Ba'al ha'Ma'or (Pesachim 18b): All agree that eating on Chol ha'Mo'ed is optional, like on Rosh Chodesh. Only Yom Tov is called Atzeres. There is a fast in which one directs his heart to Hash-m, e.g. through requesting mercy to tear a decree, like a fast due to a dream. This is permitted even on Shabbos, for this is its (Shabbos') delight, and all the more so it is permitted on Yom Tov. If one learns the entire day and he desires Torah, this is similar, and it is permitted on all Yomim Tovim, except for Shabbos, Shavu'os, Erev Yom Kipur and Purim. This is like Rava, who says that all agree that Shabbos must be also for you. Mar brei d'Ravina does not argue. The Torah did not obligate that Shavu'os be also for you. This is an attribute mid'Rabanan, because it is the day that Torah was given.

Note: It seems that the Ba'al ha'Ma'or holds that Mar brei d'Ravina did not fast on Shabbos. The Ra'avad (Kasuv Sham) says that he vowed to fast on Mondays and Thursdays, and excluded only the three days listed. Heaven forbid that he fasted on Shabbos!

Rambam (Hilchos Shabbos 30:10): Eating meat and drinking wine on Shabbos is Oneg, if one can afford it. One prays Shabbos and Musaf in the Beis ha'Keneses, returns home for the second meal, goes to the Beis Midrash to learn until Minchah, prays Minchah, then fixes the third meal on wine and eats and drinks until Motza'ei Shabbos.

Rashi (68b DH d'Ba'inan): Shavu'os must be for you, to rejoice in eating and drinking, to show that Yisre'elim are pleased with this day on which the Torah was given.

Mordechai (Shabbos 229-230): Avi ha'Ezri permits to learn and pray [until six hours on Shabbos], for we hold like R. Eliezer, who says all for you, or all for Hash-m." Mar brei d'Ravina used to fast every day [during the day] except for Shavu'os, [Erev Yom Kipur and Purim]. I later saw in Beitzah that he holds like R. Yehoshua. Avi ha'Ezri ruled like him.

Gedulas Mordechai (in Chidushei Anshei Shem, under the Mordechai): Avi ha'Ezri rules like R. Yehoshua regarding Yom Tov, but on Shabbos one may learn and pray past midday.

Poskim

Shulchan Aruch (OC 288:1): One may not fast on Shabbos until six hours.

Bach (OC 242:1): There are two Mitzvos of Shabbos. One is an Aseh to be Mekadesh Shabbos. Chachamim expounded that one says Kidush over wine. Oneg is included in this. People who learn should not say "it is good to learn the entire day and not draw our bodies to pleasures. This will save us money, and we will be Mechadesh in Torah!" R. Yehoshua requires also for you. Normally the Halachah follows R. Yehoshua against R. Eliezer, for R. Eliezer was from Beis Shamai. Seemingly, here the Halachah follows R. Eliezer. If the Halachah follows R. Yehoshua, why did Rava say "all agree about Shabbos, that we require also for you"? Surely Shabbos is no worse than Yom Tov, which must be divided. It says about Shabbos "Lem'a'an Yanu'ach." Shabbos is not No'ach (pleasant) without eating and drinking! Rather, on Yom Tov the Halachah follows R. Eliezer. Even he agrees about Shabbos. However, the Mordechai connotes unlike this. He brings that Avi ha'Ezri permits to pray and learn past midday, for we hold like R. Eliezer, and Mar brei d'Ravina used to fast almost every day. I later saw in Sukah (636) that the Mordechai holds like R. Yehoshua, and so rules Avi ha'Ezri. It seems that Avi ha'Ezri retracted to hold like R. Yehoshua. How did he initially rule like R. Eliezer regarding Shabbos? Rava taught that all agree about Shabbos!

Suggestion: Avi ha'Ezri held that Mar brei d'Ravina, who used to fast even on Shabbos, argues with Rava. He holds that R. Eliezer holds that even Shabbos must be totally for Hash-m or totally for you, and the Halachah follows him.

Rejection (Bach): He wrote "also, Mar brei d'Ravina..." This connotes that until now, he did not learn from Mar brei d'Ravina. Rather, Avi ha'Ezri initially discussed one who fasts the entire year for Teshuvah and Midas Chasidus and engages in Torah and prayer. His fast is Oneg for him. Eating would pain him, for it changes his routine. Avi ha'Ezri held that Rava would agree that such a person may fast.

Bach (DH ul'Inyan): Semag and R. Yerucham say that Shavu'os and Shabbos must be half for you. However, in the Yerushalmi R. Berachyah says that Shabbos is for Torah, i.e. the majority. Only the minority is for eating and drinking. Rava said that it must be also for you. He did not say half for you. The Rambam says that one learns until [Stam] Minchah, i.e. Minchah Ketanah (nine and a half hours, which is the majority of the day). Regarding Yom Tov (Hilchos Yom Tov 6:19), he says until midday. We hold like this. Yom Tov is half for Hash-m, and Shabbos is mostly for Hash-m, unlike Semag and R. Yerucham.

Eliyahu Rabah (242:6): R. Eliezer agrees that Shabbos must be also for you, but it need not be half for you. This resolves the Mordechai. He said "Avi ha'Ezri permits to pray and learn past midday on Shabbos because the Halachah follows R. Eliezer. I later saw that Avi ha'Ezri rules like R. Yehoshua." The Bach questioned this. I answer that he means that R. Eliezer permits fasting past midday, but not the entire day, like the Bach understood. Avi ha'Ezri retracted to rule like R. Yehoshua, and requires half for you. The Bach concluded that on Shabbos the majority must be for Hash-m. This is wrong.

Kaf ha'Chayim (242:5): The Ari Zal says that there is a Tikun through prayer, and a Tikun through eating [on Shabbos]. Even though eating and drinking on Shabbos is a Mitzvah of Oneg, one should not overeat. The Magid told the Beis Yosef to minimize eating even on Shabbos and Yom Tov. It should all be l'Shem Yichud.

Kaf ha'Chayim (6): The Bach says that Yom Tov should be half-half, and Shabbos should be mostly for Hash-m. I say so even for a Chacham. Even though he learns the entire week, and he should engage in pleasure on Shabbos, a minority suffices.

Beis Yosef (288 DH va'Yemaher): One opinion in the Yerushalmi says that Shabbos and Yomim Tovim were given only for eating and drinking. Another says that it is only to engage in Torah. The Medrash Tanchuma says that they do not argue. For people who work the entire week, Shabbos is for Torah. For Chachamim who toil in Torah the entire week, Shabbos is for Oneg. Rabah said that all agree that that we require for you on Shabbos. He means that R. Eliezer and R. Yehoshua agree. Alternatively, he refers to a small amount of Oneg, and here we discuss the entire day.

Magen Avraham (288:1): Olas Shabbos learns from Pesachim that even though Yom Tov must be half for you, Shabbos may be totally for you, as long as one does not miss the time for Tefilah. Such words should not be heard! It says also "Shabbos la'Shem"! Rather, even though R. Eliezer argues about Yom Tov, he requires also for you on Shabbos. Perhaps Olas Shabbos' text was missing the word "also".

Eliyahu Rabah (ibid.): R. Yehoshua could hold that Shabbos is better than Yom Tov, and the entire day may be for you. Olas Shabbos holds like this. There is no proof from "Shabbos la'Shem." Surely it is for another Drashah. If not, how does Rava know that R. Eliezer agrees about Shabbos? Perhaps also Shabbos must be all for you, or all for Hash-m! Also, how could v'Karasa la'Shabos Oneg, a verse from Navi, override a Torah verse? Also, the Levush (292:1) rules that Shabbos is all for you. The Beis Yosef brought so from the Yerushalmi, but says that this is for Chachamim who toil in Torah the entire week. The Tur (Venice printing), Beis Yosef and Levush omit the word "also". The Drishah (2) says that it is not in the text of the Gemara. Semag, R. Yerucham and Maharshal rule that it must be half for you. In any case, it seems that all permit Chachamim to have more Oneg than others.

Shulchan Aruch (290:2): After the morning meal, one goes to learn.

Rema: Workers who do not engage in Torah the entire week should engage more in Torah on Shabbos than Chachamim, who toil in Torah the entire week. Chachamim should engage more in eating and drinking a little, for they enjoy learning the entire week.

R. Akiva Eiger (Teshuvah 1): There is no Chiyuv Simchah on Shabbos. However, for Kevod Shabbos one must fix a meal on bread. Kavod and Oneg are not the same as Simchah. The Rambam (Hilchos Yom Tov 6:16) says that Kavod and Oneg apply to Yom Tov, just like to Shabbos, and in Halachah 17 he brings a Chiyuv Simchah on Yom Tov and Chol ha'Mo'ed.