Yom Kippur, also known as the Day of Atonement, is the holiest day in the Jewish calendar and is dedicated to repentance, reflection, and forgiveness. It is observed with a 25-hour fast, prayer, and attending synagogue services, where individuals seek to atone for their sins and start the new year with a clean slate. The day emphasizes spiritual renewal, humility, and reconciliation with oneself, others, and God.