Rabbi Eliezer and Rabbi Yehoshua disagree about how to interpret two seemingly contradictory passages in the Torah:
Atzeret te-hiyeh lakhem (Bamidbar 29:35) – a holiday it should be for you
Atzeret la-shem Elokekha ( 16:8) – a holiday for God
According to Rabbi Eliezer, a person must decide whether he wants to devote yom tov to his personal pleasures like eating and drinking or to God by spending the day learning Torah. Rabbi Yehoshua understands that a person is to divide his holiday in half – part for his own physical pleasure and part devoted to spiritual matters.
The Gemara teaches that there are some days where all agree that an aspect of personal joy and pleasure must play a role.
The amora, Rabbi Elazar says that Shavu’ot is one such day, since we are obligated to show our happiness on the day that the Torah was given.
Rabba teaches that Shabbat is such a day, since we find that the navi teaches ve-karata la-Shabbat oneg (Yeshayahu 58:13) that we must consider Shabbat a delight.
According to Rav Yosef, Purim is another such day, since we are commanded in Megilat Esther (9:22) to celebrate Purim as yemei mishteh vesimha – days of feasting and gladness.
The Gemara then relates that Mar brei d’Ravina (Mar, the son of Ravina) fasted all year, except for Shavu’ot, Purim and erev Yom Kippur. While the Gemara goes on to investigate the importance of eating prior to Yom Kippur, Mar’s behavior demands some explanation. How did he fast on the other days when there is a mitzva to eat?
Some commentaries argue that this story can only be explained if we accept that Mar brei d’Ravina followed the ruling of Rabbi Eliezer and chose to devote his holidays to Godly service, indicating that his is the position that the halakha follows. (We should note that the Shulhan Arukh, Orah Hayyim 529:1, rules like Rabbi Yehoshua.)
The Ba’al ha-Ma’or explains that we must distinguish between different fasts. A mourning fast can never be taken on Shabbat or holidays, including Rosh Hodesh and Hannuka. An incidental fast, when someone is too busy with other affairs and forgets to eat, would only be forbidden on Shabbat and Yom Tov. A fast that is connected with Torah study is forbidden on Shabbat, Shavu’ot, Purim and erev Yom Kippur.
The Ge’onim rejected this explanation entirely, explaining that Mar brei d’Ravina accepted upon himself to fast for an entire year, excluding specific days. Thus, this out-of-the-ordinary behavior cannot be used as a basis for comparison to any other situation.