With the exception of the Day of Atonement, any fast which falls on the Sabbath is postponed to Sunday. The Fast of Esther cannot be postponed to the next day since that day is the Festival of Purim, and as it cannot be predated to Friday because fasting on the day before the Sabbath is not permitted, the fast is put back to Thursday.
Laws and customs also laid down additional penitential prayers, such as AVINU MALKENU and Torah readings for certain fast days, the story of the intercession of Moses when he prayed for God's forgiveness of the people after they had sinned with the Golden Calf (Ex. 32, 34). The same passage is read both during the Morning and Afternoon services. The prophetical reading for the Afternoon Service is Isa. 55:6-56:8 which tells of God's forgiveness to the sinner who repents.
Sick people and women in an advanced stage of pregnancy, as well as nursing mothers who have recently given birth, are not required to fast. Where there is a danger to health, the rabbis ruled that fasting is not only excused but prohibited.
The Bible also records cases of individuals undertaking a private fast, usually a fast of petition. David fasted when his first child from Bathsheba was near to death (II Sam. 12:16) and the Book of Psalms testifies to individually imposed fasting (Ps. 35:13; 69:11-12). In much later periods individual fasts were undertaken or traditionally observed for one of a variety of reasons, such as to break the feared effect of a bad dream. Some mourners would fast on the day of the burial of a parent, or on the observance of a yahrzeit (anniversary) of the death. More widely observed is the custom for a bride and bridegroom to fast on the day of their wedding before the ceremony. This is in token of their desire for forgiveness for any sins as they are about to start a new life.
Various other non-obligatory fasts were observed mainly by extremely pious individuals rather than by the wide community.
For example, in the months following the festivals of Passover and Sukkot, i.e., in Iyyar and Heshvan, it became a special mark of piety to fast on the Monday, Thursday, and the following Monday after the conclusion of the festival (known as Ta'anit BeHaB). The observance is not known before the 13th century and the reason generally given is that it was a special gesture seeking atonement for any superfluous jollifications indulged in during the festival period. The same class of pietistic fasts includes Yom Kippur Katan, the "little Yom Kippur" observed on the day before New Moons. This was introduced by the 16th century kabbalist, Moses CORDOVERO of Safed, as a penitential fast for any sins committed in the previous month. Also of kabbalistic origin is the period of fasting in the winter months of January and February known as SHoVeVVIM TaT from the initial letters of the first eight weekly Bible portions of the Book of Exodus, read during this period. Some very pious Jews fast every Monday and Thursday throughout the year.
The Fast of the Firstborn on Nisan 14, the day before Passover, is the only fast which is neither an atonement for sin nor a fast of petition. It is in a class on its own and is placed in the calendar as a reminder of the death of the Egyptian firstborn and the miraculous escape of the Israelite firstborn. However, it is observed only symbolically by firstborn male Jews, with provision made to avoid the obligation of fasting by participating in a siyyum -- the study of a concluding passage of a Talmud tractate, which permits the participants to eat and drink.
In spite of, or perhaps because of, the accumulation of many additional kinds of private or communal fast days for various reasons, some outstanding rabbinic leaders opposed self-inflicted fasts and strongly criticized them. They taught that the spirituality of Judaism is not necessarily best experienced through the exercise of self-mortification, but rather through the practice of charity and good deeds.
