The mitzvah of Kavod and Oneg Shabbos:
A Biblical or Rabbinical precept: There are two matters regarding Shabbos that were expounded on by the prophets, which is [the Mitzvah] to honor Shabbos [i.e. Kavod Shabbos] and [the Mitzvah of] Oneg Shabbos. These two Shabbos obligations [to honor and enjoy it] are in truth rooted in a Biblical precept, and are included within the Biblical command to sanctify the Shabbos. However, some Poskim rule that these two above obligations of Kavod on Shabbos and Yom Tov and Oneg on Shabbos contain no Biblical root and are rather of Rabbinical origin. Nevertheless, even according to the latter opinion one must be very careful to fulfill these two obligations of honor and enjoyment on Shabbos, as Rabbinical precepts are of more severity than even Biblical precepts. [Practically, in the laws of Yom Tov Admur rules plainly like the latter opinion that the Mitzvos of Oneg and Kavod are Rabbinical precepts which have been explained by the prophets. However, according to all opinions, it is Biblically forbidden to fast on Shabbos.]
The reward: Whoever fulfills the Mitzvah of Oneg Shabbos his reward is explicitly mentioned in the Prophets that he will merit “basking in pleasure of G-dliness”. Furthermore, the sages state that whoever performs the Mitzvah of Oneg Shabbos all his sins are forgiven, and he is saved from the judgment of Gehenom.
It is disputed if Kavod and Oneg Shabbos is a Biblical or Rabbinical obligation. Nonetheless, according to all one must be very careful in fulfilling the Mitzvah, and one who does so basks in G-dliness in the future, has all his sins forgiven and is saved from the judgment of Gehenom.
Why is no blessing said over the fulfillment of the Mitzvah of Oneg Shabbos?
Various answers have been given towards this question:
Is there a Mitzvah of Simcha applicable on Shabbos?
There is no Mitzvah of Simcha explicitly mentioned regarding Shabbos.
It is clear that both on the Halachic and esoteric aspects of the Torah it is a Mitzvah to embellish in Oneg Shabbos, by eating delicacies and drinking fine beverages, and the concept of sanctifying oneself with that which is permitted does not apply on Shabbos. Nonetheless the above is contingent on that one eats and drinks the delicacies for the right intentions, which is mainly for the sake of fulfilling the Mitzvah of Oneg Shabbos. One who however does not have such intentions, but rather is simply doing so in order to fulfill his animalistic desires, such eating is no better than eating during the week, of which the concept of “sanctify yourself with the permitted” applies. Such a person is considered not to be honoring Shabbos but to be honoring himself on Shabbos. Hence it has been found that Chassidim in general as well as certain Tzadikim would diminish their amount of embellishment contained within their fulfillment of this Mitzvah. One is certainly to avoid overeating if this will refrain him from spending his time in learning Torah, which is the purpose of Shabbos.
 Admur 242:1; See M”B 242:1; Piskeiy Teshuvos 242:3; Sichas 24th Kisleiv 5732; Yagdil Torah 67 p. 355; Likkutei Sichos 4:1091
 Admur 242:1; Rambam Shabbos 30:1
The source: As stated in the verse [in Yeshaya 58:13] “/וקראת לשבת ענג לקדוש ה’ מכובדAnd one calls Shabbos enjoyment, to sanctify the honored G-d.” [Admur ibid; See also 262:3; 610:8; Likkutei Sichos 16:522]
 1st and Stam opinion in Admur ibid; Kuntrus Achron 242:1 that so is opinion of Rashi; Opinion in 250 Kuntrus Achron 2; Opinion in 288:7; Sefer Chareidim 4; Opinion in M”B 242:1; Implication of: Sefer Yereim Mitzvah 417; Ramban Parshas Emor Vayikra 23:2; Mechilta Bo 9; Sifra Emor 12; Sifri Pinchas 147; See Likkutei Sichos 11:66; Yireim 412; Tzemach Tzedek 28
The reason: As Shabbos is included within the group of days called Mikraeiy Kodesh or “A calling of holiness”, as the verse states [Vayikra 23:3] “Ubayom Hashevi’i Shabbos Shabbason Mikra Kodesh,” and the Sages [Mechilta Bo 9; Sifra Emor 12; Sifri Pinchas 147] have taught that the term “Mikra Kodesh/A calling of holiness” is coming to teach that one is to sanctify and honor the Shabbos with clean clothing, and to enjoy the day through pleasurable foods and drinks. [Admur ibid; Sefer Yereim Mitzvah 111 ; Mechilta Bo 9; Sifra Emor 12; Sifri Pinchas 147; See Mordechai Rosh Hashanah Remez 708; Likkutei Sichos 11 p. 66; 15:374 p. 528]
 2nd opinion in Admur 240:1; Kuntrus Achron 242:1 that so is opinion of Rambam 30:1; Opinion in 250 Kuntrus Achron 2; Opinion in 288:7; Rambam Shabbos 30:1; Yom Tov 6:16; See Sefer Hachinuch 297; Beis Yosef 487; P”M 242 M”Z 1; Kaf Hachaim 242:6; Opinion in Shaar Hatziyon 242:1; See Admur 487:6; 490:5; 529:5; Tzemach Tzedek O.C. 36
 The reason: As they learn that the wording of “A calling of holiness” is coming to teach that one is to sanctify the Shabbos through refraining from doing forbidden labor. [Admur ibid]
 Admur ibid; Eiruvin 21b; Yerushalmi Brachos 1:4; Shaar Hatziyon 242:1; From all the above we can deduce the great importance emphasized in fulfilling this Mitzvah which Klal Yisrael is so careful to follow, and in the wording of the Rosh, one of the greatest Rishonei Ashkenaz “The Jewish people are Adukim/fervently attached to the fulfillment of the Mitzvah of Oneg Shabbos.”
 Admur 529:5
 Opinion of the Mishneh Berurah: The Mishneh Berurah in Biur Halacha [250 “Yashkim”] sides that the eating of bread during the meal is Biblical while other delicacies are Rabbinical.
 Admur 288:7; Michaber 570:3; M”A 288:7; Rashba 4:262; P”M 242 M”Z 1 that this applies evena ccoridng to Rambam; Kaf Hachaim 242:6;However, see Kuntrus Achron 242:1 that according to the Rambam it is only a Rabbinical prohibition to fast on Shabbos
 Yeshaya 58:14
 Admur ibid; Tur 242; Levush 242; Shabbos 118a
 Shabbos 118a
 One of the explanations mentioned behind this seemingly puzzling statement is that Shabbos is a day where we reconnect with Hashem our father and king. On Shabbos the inner love between Hashem and the Jewish people is revealed and hence it has the capability to forgive and erase all of man’s iniquities. Thus, if one respects Shabbos properly, he benefits from this special relationship with Hashem and is told that his sins are forgiven, and he will be saved from the judgment of Gehenom.
 Toras Shabbos 263:7
 Toras Shabbos 263:7
 Keren Ledavid 61
 Sdei Chemed Asifas Dinim Brachos 1:16
 Admur 529:8; and so is proven from 242:1 that Shabbos does not have a Mitzvah of Simcha. However, the Rebbe in Shaareiy Halacha Uminhag 1:127 writes that the reason that the Mitzvah of Simcha was not written regarding Shabbos is because it is nullified to the Mitzvah of Oneg applicable on Shabbos. However this requires further analysis as by Yom Tov there is also a Mitzvah of Oneg [Rabbinically (529:5 as is the second opinion in 242:1), and if the Oneg involves Simcha-Biblically (242:1 KU”A 2)] and nonetheless Admur mentions also the Mitzvah of Simcha by Yom Tov. Thus, omitting the Mitzvah of Simcha by Shabbos seems to imply it does not exist, as writes Admur explicitly in 529:7.
 Iskafya is a Chassidic term used to describe self-control from indulgent in pleasures.
 For a full analysis on this see Kitzur Hilchos Shabbos Miluim p. 7; Piskeiy Teshuvos 242:3-5
 As explained above in the Shulchan Aruch
 Tanya chapter 7; Igeres Hakodesh 26; Mamarim Haketzarim of Admur Hazakein [p. 59]; Likkutei Torah Beshalacha 2a “There is no need for Avoda on Shabbos, in the same way as done during the week, which is through plowing and having a broken heart, and Iskafya. Rather Shabbos is a time for Avoda of Taanug on Hashem, and Ishapcha.”
Background: Tanya chapter 7 “One who eats fatty ox meat and drinks tasty wine…., when done for the sake of fulfilling the Mitzvah of Oneg Shabbos and Yom Tov, its divine sparks become elevated.” This is in contrast to during the week that one who eats for the sake of fulfilling his desires descends the Divine sparks to impurity. The Mitzvah is likewise stated in Igeres Hakodesh 26 “However on Shabbos that there is an elevation of the Kelipas Nogah itself together with the external aspects of all worlds, therefore it is a Mitzvah to eat all the delicacies on Shabbos and to increase in meat and wine, even though that during the week one would be considered a gluten for doing so.” This matter of difference between the eating on Shabbos and weekday is discussed in various Mamarim in Torah Oar and Likkutei Torah. [See Torah Oar Chayeh Sara 15b; Torah Oar Beshalach 65b; Siddur 200-203; Sefer Hasichos 5703 p. 142-146] In the Mamarim Haketzarim of Admur Hazakein [p. 59] he writes that in essence Shabbos is meant to be a day without eating or drinking, as it is similar to the world to come, however since it is impossible to receive the G-dly pleasure of Shabbos without a physical vessel for this pleasure, therefore one is obligated to eat on Shabbos in order to receive the spiritual pleasure which is contained within it.
 Shlah [Shabbos Neir Mitzvah]; Kesav Sofer 107:16; Kaf Hachaim 529:45; Baal Shem Tov in Keser Shem Tov 395; Sefer Hamamarim Samech Vav p. 154 “Eating on Shabbos is not a physical pleasure but a spiritual pleasure.”; Rebbe in Sichas 1951 Chayeh Sarah 18; See also Elya Raba 293:2; Reishis Chochmah Shaar Hakedusha 15:53; Mateh Efrayim Alef Hamagen 581:3
Background: Kesav Sofer 107:16 writes that one who does not eat for the sake of the Mitzvah then that meal is considered Seudas Reshus and does not contain a Mitzvah. So is also evident from Tanya chapter 7 from the words “for the sake of Oneg Shabbos”; Shlah [Shabbos Neir Mitzvah] writes: Those which eat and drink to their hearts content and due to the great amounts of foods fall into slumber are not considered to be pleasuring Shabbos but to be pleasuring themselves on Shabbos; Keser Shem Tov writes that when eating on Shabbos and Yom Tov one’s intent must be for the Taanug found in the G-dly vitality that is in the food and not the physical pleasure and one who attaches to the physical pleasure distances himself from Hashem; Sefer Hamamarim Samech Vav p. 154 “Eating on Shabbos is not a physical pleasure but a spiritual pleasure.”; Rebbe in Sichas 1951 Chayeh Sarah 18 states that even the scrupulousness of eating on Shabbos needs a measurement, and that measurement is in accordance to the amount one is scrupulous by other Mitzvos, especially Mitzvos that are painful to accomplish. To note from a story of the Baal Shem Tov who showed his students on Shabbos a man with Shabbos clothes and he appeared like an ox due to his over involvement in eating his meat; To note also from Mateh Efrayim Alef Hamagen 581:3 which writes that one may delay eating a Shabbos delicacy in middle of his meal, for the sake of Iskafya, and one who does so is considered that he has fasted the entire day. Reishis Chochmah Shaar Hakedusha 15:53 states: It is proper that one does not satiate himself with coarse foods, and he should not fulfill his desires for good foods even on Shabbos. Elya Raba 293:2 brings in name of Abudarham that one is not to eat too much on Shabbos as this will refrain him from having an appetite for the coming meal. Thus, one is to control his inclination and push away the next food even if he desires it.
 So is evident from Ksav Sofer ibid
 Shlah ibid
 Kitzur Hilchos Shabbos ibid states that this is a tradition amongst Chassidim.
 Rav Moshe, the son of the Alter Rebbe would diminish his eating throughout the week including Shabbos and Yom Tov. [Igros Hakodesh Rebbe Rayatz 7 p. 18]; Magid Meisharim end of Bo states that the Magid commanded the Beis Yosef to diminish in eating food even on Shabbos and Yom Tov.
 See Shlah ibid