A number of Midrashim indicate that the departed sentenced to Gehinom enjoy a kind of reprieve on Shabbat.
This is not brought as a mere curiosity, but on the contrary comes to explain or motivate a number of halakhot.
For example, one of the reasons given for smelling spices as Shabbat leaves is that then the fires of Gehinom are rekindled and the stench ascends to our world; the pleasant spell of the spices helps dispel the odor (Rashi, cited in Machzor Vitry).
In order to understand the concept of rest from torment on Shabbat, we have to understand the concept of the fires of Gehinom. The fires of the abyss suffered by transgressors are not meant to make them suffer per se, but rather to provide atonement for their transgressions. Our tradition states that the presence of the wicked in Gehinom is usually for a limited time only, less than a year.
Furthermore, we find in many places that these fires are in fact kindled by a person’s own sins, not by any vindictive force. Our Sages relate that the waters of the flood in the time of No’ach were boiling hot, to punish people for the sins they carried out in the boiling heat of passion (Sanhedrin 108b). And according to one opinion in the Gemara, the fires of Gehinom were kindled only on Shabbat eve, which was the time of the first sin (Pesachim 54a).
Our world is a world of significant moral ambiguity. Even a person who wants to do the right thing can often be confused into wrongdoing; furthermore, our base impulses are always tempting us into transgression. Very often the fog of uncertainty serves to assuage feelings of guilt. But when a person perceives his acts from the clear perspective of the World of Truth, he feels an intense burning remorse for all his misdeeds. This sense of shame tortures the person for a period of time until his atonement is complete.
But Shabbat is a day when the world is perfected. Since the torments of Gehinom are only transient, when the world is perfected we will be relieved of these; so it is only appropriate that on Shabbat there is rest from them.
Furthermore, Shabbat is a day when our bodily desires are elevated, so we needn’t be ashamed of them. Finally, Shabbat is a day of limited engagement with the world, including moral engagement. On Shabbat we don’t occupy ourselves with large, world- transforming undertakings which force us into painful moral dilemmas.
However, as we confront hard moral choices and troubling moral ambiguity at the start of the new week, we become acutely aware deep inside of us of the presence of rottenness in the world, and even in ourselves. This can be likened to the stench of the fires of Gehinom. We console ourselves with beautiful smells, which are the enjoyment of the soul (Berakhot 43b). We remember the many brave and righteous acts which are also performed, and the immense human capacity for good, which perfumes the world.