Question: why is yom kippur the holiest day?
Answer: Leviticus 16:29 mandates establishment of this holy day on the 10th day of the 7th month as the day of atonement for sins. It calls it the Sabbath of Sabbaths and a day upon which one must afflict one's soul. Leviticus 23:27 decrees that Yom Kippur is a strict day of rest.