"Rejoice in Hashem, you righteous"
Synopsis
The scholars decreed that praises on Shabbat should start with the psalms of David, which rise to illuminate and bless the heavens and the river that comes out of Eden. All is performed as it should be performed, though, and on this day even the sun illuminates properly.
Relevance
David signifies our physical existence. He is Malchut. Thus, David's psalms elevate our lower world (Malchut) into the heavens (Zeir Anpin) when they are recited at the commencement of each new Shabbat. For us, our own souls - and all existence - are now elevated by the sacred texts that speak these hidden truths.