Question: Is it correct that if one has a frightening dream on Friday night, one must fast on Shabbat day?
Answer: Our Sages taught (Berachot 55b) that if one has a bad dream and one is distressed as a result, one should improve it in front of three friends by the dreamer exclaiming, “I have seen a good dream” three times and the friends responding, “You have seen a good dream. It is good and it shall be good. May Hashem cause it to be good. May Heaven decree upon you seven times that it be good and it shall be good” three times. The friends recite some other verses quoted in the above Gemara to the dreamer. (Many Siddurim contain the entire order of improving a dream.) The morning after one has such a bad dream, one should fast for the entire day.
The Shulchan Aruch (Chapter 288) states that a fast to offset a bad dream may even be observed on Shabbat, for this fast serves as one’s true enjoyment on that specific Shabbat. After doing so, one must fast one more day during the following week to atone for having fasted on Shabbat.
Nevertheless, Rav Amram Gaon and Rabbeinu Klonimus write that nowadays, one should not fast on Shabbat to improve a bad dream since we are not experts in interpreting dreams to determine which dreams are good and which are bad. Although the Tur quotes their opinion, Maran Ha’Shulchan Aruch does not.
Nonetheless, Maran Rabbeinu Ovadia Yosef zt”l (in his Chazon Ovadia- Shabbat, Volume 1, page 384) quotes the opinion of Hagaon Rabbeinu Eliyahu Ha’Kohen (in his Sefer Igeret Eliyahu, Chapter 4 of Ma’aser Sheni) who writes that “although one may observe a fast to improve a dream on Shabbat, it seems that this only applies to the times of our Sages when they were experts in interpreting dreams, as opposed to nowadays. There are dreams which seem bad but are, in fact, good. Thus, I instruct anyone who asks me about fasting to better a dream on Shabbat not to fast; rather, one should enjoy himself by eating and drinking. However, one should only speak words of Torah for the duration of Shabbat, preferably read the entire book of Tehillim that Shabbat, and after Shabbat, give Tzedakah to the poor according to his means.”
Rabbeinu Mordechai Yaffeh (author of the Levush) writes in his Sefer Levush Ha’Ora (Parashat Vayeshev) that everyone knows that a vast majority of dreams are complete nonsense without even an ounce truth to them. Maran Ha’Chida writes (Ma’arechet Gimmel): “I know that a great person who was not concerned at all about dreams. He said that there was a certain Torah scholar who would spend most Shabbatot observing fasts for bad dreams. One Shabbat he came to ask me and I told him not to fast. He then stopped having bad dreams on Friday night.” (The Sefer Minchat Yehuda writes that there are certain demons that scare people in their dreams and when one becomes worried about the dream, they are happy and this empowers them to continue torturing the individual.)
Indeed, Hagaon Rabbeinu Eliezer Papo writes (in his Sefer Peleh Yo’etz) on the topic of dreams that the best thing is not to be concerned about dreams at all, not to be scared of them, or tell them over to anyone since dreams are usually nonsense anyway. This is especially true in our times when there is almost no one who is revealed anything about the future in one’s dreams and one’s dreams are usually fueled by one’s imagination and thoughts.
Although this is generally the case, there is no iron-clad rule here and every situation should be individually probed. With regards to the above question about a “scary” dream, there are many dreams that have no actual meaning and just scare the person (i.e. nightmares). It seems from the Gemara (Yoma 22b) that a frightening dream (called “Siyuta” in Talmudic terminology) is a different type of dream that has no bearing on the future at all. This is especially true nowadays, as we have explained.