Siman 571

Regarding fasts that a person is not commanded in, there is a dispute in the Gemara (11b): One says anyone who sits in a fast is called "holy." And one says he is called a "sinner." And both are the words of the living God. For certainly, if he is able to endure the fast and does not have through this a neglect of Torah study—he is called "holy," for through this his iniquities are purged. And if not, that he is not healthy and strong—he is called a "sinner." And all this is regarding one who fasts by way of asceticism. But one who is a sinner—certainly he must fast for his sins, and weep and confess before Hashem, and return and be healed; unless he will come through the fast to a state of danger, for then it is forbidden for him to fast. And so too if he is a great talmid chakham—it is better to minimize fasts, and he should increase in the study of Torah. But without this—a talmid chakham is not permitted to sit in a fast, because he diminishes the work of Heaven. And all the more so teachers of children, for there are two factors: neglect of the Torah study of the children of the house of their teacher, and he robs the employers. And so too any laborer is not permitted to increase in fasting, because he diminishes the work of the homeowner. However, regarding public fasts—talmidei chakhamim and teachers and laborers are obligated to fast, and it is forbidden to separate oneself from the community. And with this understanding they accepted them. And all those occupied with the needs of the public—their law is like teachers. And regarding the matter of fasting, there is the law of a talmid chakham even in this time. And a minor boy and a minor girl when they take upon themselves a fast—their law is like vows, and it was explained in Yoreh Deah siman 234, see there.

There are times when they are obligated and need to decree a fast, and it is impossible. For example, if it was decreed upon them that they may not decree a fast, or for example, if there is a plague and it is impossible to decree a fast because there is danger in fasting during a time of plague. Or a city surrounded by oppressors, where it is impossible to frighten the inhabitants of the city by decreeing a fast. And likewise an individual pursued by an oppressor, or upon whom rests an evil spirit, and he is unable to fast now. What shall they do? They should accept upon themselves such and such fasts for when they are saved, or when the decree passes.

The books of Musar wrote that one who desires to eat and eats, and in the middle pulls his hand back from his desire—it is a great thing, and a great repentance, and it is considered an asceticism (Magen Avraham, end of subsection 1).