Yom Kippur is the holiest day in Judaism, and it is often referred to as the Day of Atonement. It's a reflective holiday focused on repentance, forgiveness and spiritual renewal. It marks the culmination of the Ten Days of Repentance, which began with Rosh Hashanah, the Jewish New Year. It begins at sunset on Wednesday, Oct. 1, and ends at nightfall on Thursday, Oct. 2.