As we approach Yom Kippur, we recognize that it is unquestionably one of the most important days of the year. And yet, in many ways, it is a mystery. While one might assumedly categorize it as a day of suffering and sadness, Chazal refer to Yom Kippur as a spiritually uplifting day of atonement and rebirth (Taanis 4:8). There is even an element of the day that is associated with the happiness of Purim (Yom K’-Purim, a day like Purim). At the same time, though, it is a fast day. We normally characterize fast days as times of mourning and sadness, such as Shiv’ah Asar B’Tamuz and Tish’ah B’Av. How is Yom Kippur different, and what is the nature of this day?