Rav Yehuda said in the name of Rav: Whoever delights in Shabbat is given the desires of his heart, as it is said: "Delight in Hashem, and He will give you the desires of your heart." What is this delight? When it says, "And you shall call the Shabbat a delight," it refers to the delight of Shabbat. How does one delight in it? Rav Yehuda, the son of Rav Shmuel bar Shilat, in the name of Rav, said: With a dish of spinach, large fish, and heads of garlic. Rav Chiya bar Ashi said in the name of Rav: Even a small thing, if made for the honor of Shabbat, is a delight. What is it? Rav Pappa said: Small fried fish.
Neither the Shulchan Aruch nor the Rambam mentions this. The Rambam says four things were said about Shabbat – two from the Torah and two from Sofrim, and they are explained by the Nevi’im. The Torah says זָכוֹר וְ שׁ ָ מ ו ֹ ר – Remember and Observe, and the prophets present it as Honor and Delight, as it is said: וְקָרָאתָ לַשַּׁבָּת עֹנֶג וְלִקְ דוֹשׁ ה' מְכֻבָּד – And you shall call the Shabbat a delight, and the holy of Hashem honored. The Rambam then expands on “delight”:
אֵ יזֶהוּ ֵ ן תּ ַ ב ְ שׁ ִ י ל שׁ ָ מ ֵ ן ב ּ ְ י ו ֹ ת ֵ ר ... The delight that the Chachamim spoke of refers to preparing a particularly fatty dish and a spiced drink for Shabbat, each according to one's means. Whoever increases their expenditure for Shabbat and prepares many and good foods is praiseworthy.
If this is the case, I will try to bring evidence for the words of the Shulchan Aruch HaRav, who states that eating fish on Shabbat is an absolute obligation from the Torah. The Midrash discusses what happens when Purim falls on Shabbat, and says (Tanchuma, Bereishit 3):
This is what we have learned: The Megillah is read, etc., for one who sits in fasting, as it is forbidden to fast on Shabbat. If the fourteenth falls on a Sunday, it is forbidden to fast on Shabbat. It is also forbidden on Friday due to the preparations for Shabbat, so they advance and fast on Thursday, which is the eleventh of Adar. If the fourteenth falls on Shabbat, it is forbidden to fast on Friday because of the preparations for Shabbat. The essence of fasting is forgiveness and mercy, but it should not interfere with the honor of Shabbat. The honor of Shabbat is preferable to a thousand fasts, as the honor of Shabbat is from the Torah, while fasting is rabbinic.
Perhaps the Shulchan Aruch HaRav derived the matter of eating fish as being a Torah obligation from here; since the Gemara asks "How does one delight in it?" and answers "with large fish," and the Midrash here adds and says that the delight of Shabbat is from the Torah. Perhaps this is the source for his words.
Rabbotai, why eat fish at all? Everyone will immediately quote the line that appears in many Midrashim and Sifrei Derash: הָאוֹכֵל דָּג בְּיוֹם דָּג נִצּוּל מִדָּג – He who eats fish on the day of fish is saved from fish; playing on the word דָּג and its numerical value (7) to teach that one who honors Shabbat – the seventh day – by eating fish, draws protection from judgment or misfortune – יהִ נּ ֹם'גֵּ ּין 'דִ.
The sefer Kerem Shlomo says not only that he who eats fish on the day of fish is saved from fish, but also one who eats מר"ק (soup) – which equals 7 in gematria ketana – on the day of ד"ג is saved too. And so too בש"ר (meat) and חל"ה (challah) which also equal 7. So, if we go by the number 7, everything fits except for one thing: חמין – cholent. Maybe removing the beans will make it work out!
The Bnei Yissaschar, in his Ma’amarim on Shabbat (1:11), explains the deeper reason for the custom of eating fish on Shabbat. He writes that the word שבת has the numerical value 702, which equals three times ברכה (227) plus the letters יה"ו (3 x 227 = 681 + 21 = 702). This teaches that Shabbat embodies a threefold blessing joined with the sanctity of the Divine Name. He then connects this to the pattern of blessings in the Creation story. Blessing was bestowed on three consecutive days: upon the fish on the fifth day, upon man on the sixth, and upon Shabbat itself on the seventh. This idea is hinted at in the Pasuk: בִּ נְאוֹת דֶּ שׁ ֶ א יַרְ בִּ יצֵנִי, whose initials form דם'בת, א'גים, ש 'ד – fish, Shabbat, man. As Kohelet says, a threefold cord is not quickly broken, thus, one who is called “adam” – i.e., a member of Klal Yisrael – who eats fish, in honor of Shabbat, draws upon that very threefold blessing of Creation: the blessing of the fish, the blessing of man, and the blessing of Shabbat itself.
The Bnei Yissaschar continues, quoting the students of the Arizal who wrote to recite רֹעִי לֹא אֶחְסָר 'מִ זְמוֹר לְדָ וִ ד ה even on weekdays before the meal, because it contains 57 words, as it nourishes and sustains all. It also contains letters that equal זכ"ר in gematria, which equals ברכ"ה, to send blessings in all our deeds. This seems to be the reason Chazal instituted saying this Tehillim three times at Seudah Shleesheet, based on the reasoning for eating fish on Shabbat and the three blessings given on three consecutive days of Creation.
