Yom Kippur—the Day of Atonement—is Judaism’s most solemn holy day, but its enduring power lies not in sadness. It lies in the radical proposition that human beings can stop, take account and begin again. Observed on the 10th day after the beginning of the Jewish calendar, it marks the end of the “Days of Awe,” the Ten Days of Repentance that begin with Rosh Hashanah, the Jewish New Year. The day’s scriptural roots are ancient, but its central qu…