888. If the Reason to Fast Goes Away

127:16 If a person ate impermissibly on a personal fast day, whether he did so accidentally or on purpose, he must complete the fast afterwards, and he must also fast after this on Monday, Thursday and Monday as an atonement for eating on the fast day. If he was fasting because of a vow, he certainly must fulfill his vow afterwards. (If he ate less than an olive-sized portion, he must complete this fast but he need not fast again to compensate – Mishnah Brurah 568:5.)

127:17 If a person was fasting because of some personal trouble and it passed, or if he was fasting for a sick person who either recovered or died, he must complete all the fasts that he took upon himself (unless he specified a condition otherwise. Similarly, if one commits to give charity should a sick person recover and that person dies, he no longer has to donate the money – MB 569:4). Similarly if one took fasts or some other mitzvah upon himself until his son’s bar mitzvah, if the son should happen to die before this, the father must still carry out his vow until the time when his son would have become bar mitzvah. However, if one was informed before committing to fast that the reason for doing so has already passed, he would then be resolving to fast in error and he would not have to complete the fast. (If he was informed after committing but before starting the fast, he must still fast – MB 569:7.)