http://revach.net/article.php?id=2808

Section: Shabbos   Category: Likras Shabbos
Shabbos's Sweat Instead Of Tears
“The sweat from the exertion of preparing for Shabbos has the same attribute as tears of regret – it wipes away all the aveiros one performed during the week.  The harder one exerts himself on preparing for Shabbos, the more he gains." (Shaarai Teshuvah 250:2)

Shabbos is intrinsically connected to teshuvah.  The goal of teshuvah is not only to regret our sins – but to regain our closeness to Hashem, which was lost through our aveiros.  Shabbos is the ultimate vehicle of drawing close to Hashem, and therefore contains the inherent power of teshuvah.  This is why a Korban Chaatas is not brought on Shabbos (Rambam)    The Chidah writes that the letters in v’Shabbos also spells out V’shavta – You will return.  We return to our Souce on Shabbos - the power of teshuva is inherent in Shabbos.   
 
Rav Chiya bar Abba (Shabbos 118a) says that anyone who keeps Shabbos properly, will be forgiven for all his sins.  This statement may explain the behavior of Adam before the first Shabbos of the world. The Midrash says (Berashis Rabba 22;28) that after Kayin was punished, he met Adam Harishon who inquired what his judgment had been.  Kayin told Adam that he had done teshuvah and appeased Hashem.  Adam answered that he had not been aware of the power of teshuvah.  He then immediately began reciting, “Mizmor shir leyom Hashabbos.” (Tehillim 92:1)  A song about Shabbos is a fitting ode for teshuvah.