In the past, many people fasted on Erev Rosh Ha-shana, to accept suffering for their sins, for when a sinner accepts suffering as part of repentance, he is granted atonement and exempted from more severe punishments warranted by his sins. The same applies to public fast days; they atone for sins and exempt the community from further punishment. The Sages offer a parable:
To what can this be compared? To a city that owed a large amount of tax to the king. The king sent agents to collect it, but they were unsuccessful. The city could not pay because the debt was so large. What did the king do? He told his servants and soldiers, “Let’s go there!” By the time the king and his entourage had traveled ten parasangs, the residents heard of his journey, and were frightened. What did they do? Their leaders went out to greet the king. He asked them, “Who are you?” They replied, “We are residents of such and such a city, the one to which you sent tax collectors.” He asked them, “What do you want?” They replied, “Please do us a kindness, as we have nothing to give.” He said to them, “For you, I will reduce the amount by a third.” When the king got closer to the city, ordinary residents went to greet him. He asked, “Who are you?” They responded, “We are people from such and such a city, to which you sent tax collectors, but we cannot pay. We ask that you take pity on us.” The king reduced the payment by another third. He got even closer, and all the residents came out to greet him, young and old. He asked them, “What do you want?” They responded, “Our master and king, we are not able to pay what we owe.” He forgave the final third.
The king in this parable refers to the King of kings, the Holy One, blessed be He. The people of the city are the Jews, who accumulate sins all year long. What does God do? He tells them, “Repent, starting on Rosh Ha-shana.” What do they do? On Erev Rosh Ha-shana the leaders of the generation fast, and God forgives a third of the sins. From Rosh Ha-shana until Yom Kippur, individuals fast, and God forgives another third of the sins. On Yom Kippur, all Jews fast and beg for mercy – men, women, and children – and God forgives them completely. Thus we read (Vayikra 16:30), “For on this day, atonement shall be made for you to purify you of all your sins; you shall purify yourselves before the Lord.” (Tanḥuma Emor §22)
Since the Sages said that fasting before Rosh Ha-shana is very effective, most Jews in the medieval era fasted on that day (see the next section). So states Shulḥan Arukh: “It is the custom to fast on Erev Rosh Ha-shana” (581:2). Since the fast is not mandatory (as are the fasts which commemorate the destruction of the Temple), many fasted only half the day or until plag ha-minḥa (1.25 seasonal hours before sunset). They did not fast the whole day so as not to start the holiday in a state of deprivation (Rema ad loc.; MB 562:10).
Some people fasted for ten days, as Rema writes (ad loc.): “Those who are meticulously observant customarily fast for ten days, and it is proper to do so.” They would fast for six of the Ten Days of Repentance (as it is forbidden to fast on the two days of Rosh Ha-shana, Shabbat, and Erev Yom Kippur), as well as four days prior to Rosh Ha-shana.
In recent times, far fewer people follow customs that involve fasting; even on Erev Rosh Ha-shana, most do not fast. Some suggest that people are weaker and softer than they used to be, so it is not fair to demand that they deprive themselves as an expression of piety (Ḥayei Adam 138:1). The great Hasidic masters maintained that the primary mode of worship in our generations should be joyful, so customs that detract from joy should be avoided. If someone would like to maintain the custom of fasting on Erev Rosh Ha-shana, but he finds it difficult, he should give charity instead. The amount should be either what he would be willing to pay to avoid needing to fast, or minimally, what he would spend for food on an ordinary day.