R’ Aryeh Finkel (As seen in the Lekach Tov on Shabbos) says that Shabbos is a time for Teshuva.
The Midrash Rabba on Bereishis says that after Kayin killed Hevel, He had the following encounter with his father Adam HaRishon:
Adam HaRishon: “How did your case turn out?”
Kayin: “I did Teshuva and was spared”
Adam HaRishon then proclaims: “This is the power of Teshuva, and I had not known (until now)! He then said “????? ???, ???? ????.”. We need to understand the connection between this psalm, and teshuva. When looking at this psalm, it does not appear to have any mention of teshuva. Additionally, other than the 1st line, what does it have to do with Shabbos?
We can answer this from a story with R’ Sa’adia Gaon. R’ Sa’adia Gaon says he learned the Inyan of teshuva from a simple individual. R’ Sa’adia Gaon was once due to arrive in a specific town, and that town held a lottery as to who would meet and escort the Rav during his stay in this town. The lottery was won by a simple man, who did not know the gadlus of R’ Sa’adia Gaon. Nevertheless, he was respectful to the Rav and fulfilled every request the Rav had. When he discovered the true gadlus of R’ Sa’adia Gaon, he apologized and requested mechila, for had he known the gadlus of the Rav, he would have honored him much more.
R’ Sa’adia Gaon says that we learn teshuva from this person. A person does teshuva for recognizing that he did not honor, respect or serve HaKadosh Baruch Hu properly. Behold, now he better realizes the Gadlus of Hashem, and thus he acknowledges that he should have served Hashem better than he has until now. Everyday a tzaddik has a greater realization of the Gadlus of Hashem, and thus he realizes that his servitude until now has been lacking. Even more so when one recognizes that Aveiros have been committed, this shows a lacking in one’s acceptance of the Gadlus Hashem, and when one is able to recognize this Gadlus, ultimately a teshuva needs to be done to repair the previous flawed servitude.
So, what is the connection to Shabbos? On Shabbos we have a Nishama Yiseira and (as the Ibn Ezra says) we have a greater ability of recognition. Therefore, through Shabbos, one can reach a higher level of Emuna (and this is inbuilt within the Shabbos, from Ma’aseh Breishis). Then one is awakened to do teshuva, since he realizes that his service of Hashem until now wasn’t 100%.
Now we can see how this psalm works (even without explicitly talking about Shabbos, or Teshuva). Adam HaRishon saw the power of teshuva, and said ????? ???, ???? ????. Shabbos completes the creation (of a person), and with this power that Shabbos adds, one has a greater lever of understanding the Koach and Gadlus of Hashem, and this brings him to the aforementioned teshuva.
This also explains why the psalm only discusses praise of Hashem. For when Shabbos enters a person (and completes him) he then sees the greatness of Hashem and he says: ?? ?????? ?’ ?????; ????? ???? ???? (For you make me glad by your deeds, Hashem;
I sing for joy at the works of your hands.) Through this happiness we are able to do teshuva.
This is how we see that the psalm ????? ???, ???? ???? discusses both Shabbos and Teshuva without ever mentioning Teshuva!