The Gemara on amud Beis tells us that though one should fast for a troubling dream, even on Shabbos, he still needs additional forgiveness for fasting on Shabbos. Therefore, he must fast an additional fast in order to obtain forgiveness for having fasted on Shabbos.
What does this mean and how can we understand the idea of deliberately doing something in order to achieve forgiveness, and then needing forgiveness for the act itself? Tzidkas Hatzaddik (128) compares this to an Aveirah Lishmah. That is, there exists an idea that sometimes a given act can be sinful but also necessary and can be done with pure intentions. He brings as an example Queen Esther’s sexual activity with Achashveirosh, as well as Moshe breaking the Luchos. Interestingly, he says even an Aveirah Lishmah requires forgiveness, as we see from the idea that one must fast afterwards for the fast he had on Shabbos.
Tosafos here elaborates further. He explains that the overall imperative makes the deed worthwhile, though there are aspects of it that still are sinful and require forgiveness. He compares this to the Nazir whose excessive abstention is considered sinful, and yet when done with the proper motivation, we say the benefits outweigh the sin, and it is still a good thing to do.
Reishis Chokhma (Gate of Humility 6) quotes a Zohar II:207b that elaborates specifically in regard to fasting on Shabbos. The Zohar explains that the pleasures of this world are occluded with a satanic overlay which is only removed on Shabbos. Thus, the pleasures one experiences on Shabbos are spiritual, however the pleasures experienced during the week have spiritual contamination.
Therefore, if one abstains from pleasure on Shabbos and then at the same time takes an equal pleasure during the week, it is particularly unbalanced and toxic. The Zohar compares this to the difference between a ganav (thief who conducts himself in secret) and a gazlan (bandit who conducts his treachery openly). The Torah punishes the ganav with double fines, while the gazlan only pays for the theft itself. The reason is because the gazlan makes no additional disrespect toward God, as clearly he is simply without any sense of shame. However, the ganav obviously has a sense of shame, and yet he is not ashamed before God. Similarly, it’s one thing to abstain from pleasures of the world on shabbos AND weekday, because this is not inherently disrespectful to the Shabbos. However, if one only fasts on Shabbos and not during the week, then it is disrespectful toward Shabbos specifically. This is why we must fast an additional fast during the week, to rebalance the equation.
Lomdus aside, the overall psychological message is that we cannot fully escape the consequences of our actions, no matter how well-intentioned they are. This is an important lesson because there are times where we must do things as parents or spouses that are difficult, and even inadvertently hurtful. This serves as a humble reminder that despite any justifications we may have, we might still need to ask for forgiveness.
Translations Courtesy of Sefaria, (except when, sometimes, I disagree with the translation .)