Forty days after they received the Torah at Mount Sinai and committed to be G‑d’s chosen people, the Children of Israel worshipped a Golden Calf. Moses pleaded with G‑d not to destroy His errant nation, and on the 10th of Tishrei G‑d said, "I have forgiven." Ever since, we observe this date as a "Day of Atonement"—a celebration of our indestructible relationship with G‑d. It is the holiest day of the year, when we reconnect with our very essence, which remains faithful to G‑d regardless of our behavior.
Yom Kippur is a fast day: from sundown on the eve of Yom Kippur until the following nightfall we do not eat or drink (if you’re ill, consult a rabbi). We also abstain from certain physical pleasures: wearing leather footwear, bathing or washing, applying lotions or creams, marital relations. It is also a "day of rest" on which all work is forbidden (as on Shabbat).
We wear white clothes in emulation of the spiritual angels, and spend the greater part of the day in the synagogue engaged in repentance and prayer. There are five prayer services: 1) The evening prayers, which begin with the solemn Kol Nidrei. 2) Morning prayer. 3) Musaf, which includes a description of the Yom Kippur Holy Temple service. 4) Afternoon prayer, wherein the Book of Jonah is read. 5) Ne’ilah, recited as the day wanes and the verdict for the new year is sealed. The first four prayer include a (private) confession of sins to G‑d.
Many laws and customs are associated with the prayer services; your synagogue rabbi will lead you along as needed.
End of the Fast
Ne’ilah concludes with the congregation calling out the Shema in unison, and then a blast of the shofar signals the end of the day.
Yom Kippur is followed by a festive meal. We rejoice, confident that G‑d has forgiven our sins.