We all seek remedies for our iniquities, potions that will wipe our slates clean and allow us to start afresh. Our sages advise us of the strongest elixir for our sins: The heavenly court tears up an evil judgment of 70 years for anyone who fasts on Shabbat (B. Berachot 31b). Thus sins committed during our entire lives - 70 years in this world - are erased as if they never were, thanks to a Shabbat spent abstaining from food and drink. Why might someone choose to fast on Shabbat? The commentators offer two explanations for this powerful tonic. The popular and oldest approach suggests that our sages are referring to a fast undertaken after a foreboding dream (quoted in the name of Rav Hai Gaon, 10th-11th centuries, Pumbedita). Indeed, elsewhere in the Talmud we are told that fasting effectively incinerates an ominous dream much as fire consumes the flammable tow of flax (B. Shabbat 11a). Our sages add that the fast is particularly effective on the day following the dream, even if this day is the holy Shabbat. Later authorities limit the fasting following a bad dream to particular circumstances, such as being visited by the same dream thrice. Alternatively only certain dreams are ill-omened, and some examples include: dreaming of a Torah scroll being burned, picturing the final moments of Yom Kippur, seeing the beams of your house crashing down or your teeth falling out (though not your cheeks falling for this a promising sign), dreaming of reading from the Torah or seeing your wedding in a dream. Other authorities counter that in our time we do not really understand dreams and therefore cannot ascertain whether the dream is an evil premonition or a favorable omen. As a result, nowadays fasting on Shabbat is generally not a recommended course. (Shulhan Aruch OH 288:5). Returning to the talmudic passage, another approach suggests that since our sages do not mention a word about dreams here, we must be talking about another type of fast: a fast undertaken in repentance for a particular sin. The faster desires to return to the Almighty and out of deep regret for the crimes committed, he fasts on the day when the community rejoices on Shabbat (Rabbi Betzalel Ashkenazi, 16th century, Egypt-Jerusalem). Relying on the silence of our sages in this passage, however, is not so clear cut, for a third talmudic account clearly connects between fasting for dreams and the requirement to compensate a Shabbat fast (B. Ta'anit 12b). How does the Shabbat fast panacea work? Why is fasting on Shabbat more potent than a penance undertaken on any other day of the week? Shabbat is a time when we all joyfully celebrate. One expression of this cheery feeling is in the culinary department: An extra meal is served and delicacies procured during the week are saved for Shabbat. In this communal festive atmosphere, depriving oneself by fasting becomes more challenging than usual. This undertaking is acknowledged as complete regret and results in revoking evil decrees (Rashi, 11th century, France). This magical elixir, however, is fraught with problems. After stating the power of fasting on Shabbat, our sages immediately issue a dire warning: Despite the merit of such abstention, the heavenly court returns and exacts punishment from the faster for the infringement of neglecting oneg Shabbat, the requirement of delighting on this holy day. Confounded by this paradoxical state, the Talmud asks: How can the sin of disregarding oneg Shabbat be rectified? The remedy is fascinating: The Shabbat faster must observe an additional fast to atone for the Shabbat fast. If the Shabbat fast constitutes an infraction of oneg Shabbat, a violation so serious that it requires another fast to atone for the first, why is it allowed in the first place? One commentator explains that for the individual fasting, abstaining from eating on Shabbat actually constitutes oneg Shabbat, for the fast acts as a calming tonic for the troubled soul, quelling fears of the foreboding dream or of imminent retribution. Nevertheless, the faster has not celebrated Shabbat in the proper oneg Shabbat spirit and therefore must make amends with an additional fast (Rashba, 13th century, Barcelona). Another commentator adds an insightful dimension: If God sent this ill-omened nightmare on Friday night, forcing the dreamer to violate oneg Shabbat, it is probably because the dreamer was not scrupulous about oneg Shabbat previously. The nightmare, resultant Shabbat fast and subsequent penitential fast should awaken dreamers to the fact that they have not been rigorous in their oneg Shabbat observance (Rabbi Ya'acov Reisher, 17th-18th centuries, central Europe). When should this additional fast be observed? The commentators suggest that it should be held on the Sunday immediately following the Shabbat spent without eating (Rashi). The same logic that led to fasting on Shabbat - an urgent need for atonement - guides the timing of the second fast. Moreover, immediately fasting ensures that the penance is immediate and publicly demonstrates that the supplementary fast is aimed at atoning for the breach of oneg Shabbat. Fasting on the following Sunday, however, is not obligatory and if such a course would be too physically taxing, the additional fast can be delayed (Rashba). Indeed a public fast - with the exception of Yom Kippur - that falls on Shabbat is not observed on Shabbat; it is either brought forward to the preceding Thursday as in the case of the Fast of Esther, or delayed until the following Sunday as in the case of Tisha Be'av, the 17th of Tammuz, Fast of Gedalya and 10th of Tevet (Shulhan Aruch OH 550:3). Shabbat is not something to be trifled with. Its sanctity is enshrined in our tradition. When personal circumstances necessitate a digression from the Shabbat atmosphere, we must realize that there is a price to pay. Thus our sages require a compensating fast in lieu of the fast on Shabbat. Though a Shabbat fast can atone for life sins, such action breaches the sanctity of Shabbat and therefore is considered problematic. This duality displays the unique potency of Shabbat. The writer is on the faculty of Pardes Institute of Jewish Studies and is a rabbi in Tzur Hadassah.