Q. Since Rosh HaShanah is so solemn, why don’t we fast?
A. There must have been an ancient custom of fasting, since both the Babylonian and Jerusalem Talmuds mention it. However, it gradually waned, at first by being limited in its application.
There is a Midrash in P’sikta d’Rav Kahana that reports, “On Erev Rosh HaShanah the leaders of the congregation fast and God forgives them a third of their sins. From Rosh HaShanah to Yom Kippur the scholars fast and they are forgiven a third of their sins. On Yom Kippur everyone fasts.”
The great medieval authorities, the ge’onim, opposed the custom of fasting on Rosh HaShanah, and Sa’adia Ga’on actually forbade it, saying that Rosh HaShanah is a festive day on which one should eat. He found Biblical support in a verse in Nehemiah (8:10), “Go your way, eat the fat and drink the sweet… Do not grieve, for the joy of the Lord is your strength.”
Nonetheless there was an Ashkenazi custom of fasting (Machzor Vitry 345; Tur 562:2), but this was later abandoned. However, there was still a widespread custom to fast on Erev Rosh HaShanah, and some claim that the fast of Gedaliah (following the two days of Rosh HaShanah) recalls the old custom.
There is certainly something to be learnt from the verse we have quoted from Nehemiah which says, “Send portions to him for whom nothing is prepared”, which implies that our rejoicing is diminished unless we enable others to keep the festival.