Those who observe Shabbat and call it a delight {See Isaiah 58:13-14} shall rejoice in Your kingdom. The people that hallows the Seventh Day: they will all be satisfied and delighted with Your bountiful goodness. Because You were pleased with the Seventh Day, You hallowed it, calling it “most precious of days, a remembrance of Creation.”
The Jewish worship cycle consists of three services on weekdays:
On Shabbat and Festivals, a fourth service is added, reflecting the additional sacrifice which used to be offered in the Jerusalem Temple. It is called מוסף musaf, from the root ס ף י “to add”.
In order to qualify as an official “service,” there must be a recitation of the tefila. Yismechu is an additional paragraph in the kedushat hayom prayer in the Shabbat musaf service. It reminds us that observance of Shabbat is not a burden but rather a source of joy and delight. During the cantor’s repetition of the musaf tefila, this paragraph is typically sung with gusto, the cantor chanting each verse in turn, and the congregation returning to the opening verse Yismechu v’malchut-cha shomrei Shabbat v’korei oneg as a refrain.