Rosh Hashana and Yom Kippur go together. In rabbinic tradition, they are the Yamim Noraim, the Days of Fear; a ten-day period of divine judgement. Next to the Sabbath, they became the most important holidays of the Hebrew calendar.
Their origins are obscure. Rosh Hashana (which means “new year”) began as a day of fearful appeasement of the rain god, a time of judgment for agriculture and group survival. Although it may have initially followed the autumn festival of Sukkot (just before the beginning of the rainy season), Rosh Hashana ended up on the first day of the Hebrew month of Tishri. In the minds of natives, it was a scary day with threats of drought hanging over everyone’s head. Solemn suffering was appropriate behavior. A shepherd’s ram’s horn, a primitive trumpet of warning, was blown to keep anxiety high.
In the Torah, Rosh Hashana is not the new year celebration. Tishri (September-October) is the seventh month, not the first month. Since the new moon of the seventh month was a day of dangerous supernatural intrusion, the priests designated the day a time of warning, a Yom Truah, with shofar blasts for admonition.
In early times, Yom Kippur most likely preceded both Sukkot and Rosh Hashana. It was a day of purification when sacred places and temples were cleared of supernatural defilements so that subsequent celebrations could proceed without peril. In the Torah, it follows Rosh Hashana and is the one day of the year when the high priest confronts Yahveh face to face through a screen of incense smoke to plead for the forgiveness of communal sins and to ask for reconciliation.
In rabbinic Judaism, Rosh Hashana and Yom Kippur are elevated to supreme importance, replacing the old stars of Sukkot and Pesakh. The constitute the beginning and the end of the Yamim Noraim, the Days of Terror. The seventh month has become the first month. Yom Truah has become the New Year.
The theological rationale for the new structure is imposing. An omnipotent Yahveh, no promoted to world domination, undertakes an annual judgment of humanity, especially of his chosen servants, the Jews. Descending from heaven, he enthrones himself in Jerusalem and for ten days sits in judgment, rendering his initial verdict on Rosh Hashana and his final verdict on Yom Kippur. In the face of his terrifying presence and power, the safest human response is to appear as pitiable as possible. Temporary starvation, torn clothing, weeping, and obsequiousness are effective procedures for arousing pity and reducing divine anger. Ina sense, the annual judgment is a preview of the final judgment of the Resurrection Day when the living and dead will confront their ultimate destination.
from Judaism Beyond God
Rabbi Sherwin Wine
Milan Press, 1995