Shabbat is to be observed as a day of "Oneg" – enjoyment. Our Sages teach that everything we do on Shabbat must be done in a special fashion, differently than the way it is done during the week. We eat differently on Shabbat than during the week, we speak differently, and we dress differently.
The Sages also teach that one who fulfills the Misva of "Oneg Shabbat" – enjoying Shabbat – is rewarded with a "Nahala Beli Mesarim," or "portion without borders." The verse says in the Book of Yeshayahu (58:14), "Then you shall delight in Hashem, and I will elevate you upon the heights of the earth, and I will feed you the portion of your patriarch, Yaakob." The reward for "delighting in Hashem," enjoying oneself in honor of Shabbat, is the "portion" of Yaakob Abinu. Our Sages note that the promise given to Yaakob differs from the promises given to Abraham and Yishak. When God promised Eretz Yisrael to Abraham Abinu, He told him, "Arise, and walk through the land, to its length and width, for I am giving it to you" (Bereshit 13:17). Abraham was thus promised only the land within the borders. Yishak was similarly promised that his descendants would receive "all these lands" (Bereshit 26:4) – a delineated territory. To Yaakob, however, God promised, "You shall spread forth to the west, to the east, to the north and to the south" (Bereshit 28:14), implying that his descendants would expand indefinitely.
Thus, when God promises to grant "the portion of Yaakob" to those who enjoy the Shabbat, it means that they will receive a reward without boundaries. This has been explained to mean that they will enjoy all the benefits of this world as well as great reward in the next world.
The Shela Ha’kadosh (Rabbi Yeshaya Horowitz, 1565-1630) noted that the term used in reference to this Misva is "Oneg Shabbat" ("enjoying Shabbat"), and not "Oneg Asmo" ("enjoying oneself"). Meaning, the enjoyment of Shabbat must be done genuinely for the sake of Hashem, and not for personal gratification and indulgence. Only then, if a person gives honor to Shabbat by sincerely observing it as a day of enjoyment, then he will receive the special reward assigned to those who "delight in Hashem."
