This "Day of Atonement" actually starts at sundown on the previous day. (The Jewish day runs from sundown to sundown.) It is a very solemn day in the Jewish year that is devoted to fasting, prayer, and repentance. On Yom Kippur, people repeat a chant called the "Al Chet" (Chet is usually translated as "sin"). In the chant, the people list the ways in which they have fallen short over the past year and repent of these shortcomings.