On today’s daf, the rabbis continue to discuss the laws of personal fasts. While most of Tractate Taanit concerns public fasts that were called in response to some kind of communal emergency — most notably lack of rain — individuals could also choose to undertake personal fasts. Though this practice has largely been abandoned, in rabbinic times these fasts could be observed for any number of reasons, including as a gesture of repentance, for spiritual cleansing, or on the yahrzeit of a loved one or a teacher.
They could also be undertaken in case of a bad dream, a fast known as a taanit halom — which literally means (you guessed it) “dream fast.” The rabbis believed that dreams could convey profound truths, even messages from another realm. A disturbing dream could be a portentous sign of things to come and fasting was seen as a method of averting whatever evil fate might be in store.
And so we encounter this story on today’s daf:
Rav Yehoshua, son of Rav Idi, happened to visit the house of Rav Asi. They prepared a third-born calf (whose meat is high quality) for him.
They said to him: Let the Master taste something.
He said to them: I am observing a fast.
They said to him: And let the master borrow and repay the fast. Doesn’t the master hold in accordance with this halakhah that Rav Yehuda said that Rav said: A person may borrow his fast and repay?
Rav Yehoshua, son of Rav Idi, said to them: It is a fast for a dream.
The context for this story is a teaching from Rav Yehuda in the name of Rav that one may “borrow and repay” a fast — that is, if someone cannot fast on the day they intended to, they simply annul the fast and fast on a different day. The story of Rav Yehoshua suggests that a taanit halom is an exception to this rule.
The Gemara goes on:
And Rabba bar Mehasseya said that Rav Hama bar Gurya said that Rav said: A fast is effective to neutralize a bad dream like fire is effective for burning chaff.
Rav Hisda said: The fast is effective specifically on that day that one dreamed.
Unlike other types of personal fasts, a taanit halom must be undertaken on the day one had the dream. It can’t be delayed. This practice is so important, in fact, that it’s even permissible to observe a taanit halom on Shabbat. Enjoying Shabbat is so sacred that every fast day (save for Yom Kippur) is bumped forward a day if it falls out on Shabbat. But if someone had a frightening dream on a Friday night, this rule is suspended and a personal fast is permitted.
So how does one make up for desecrating Shabbat by fasting? Easy, the Gemara tells us:
Let him sit in observance of another fast, on another day, to atone for his fast on Shabbat.
Read all of Taanit 12 on Sefaria.