Yom Kippur is a critical day in the Jewish calendar. The sabbath (Shabbat) is the most important holiday, but Yom Kippur is sometimes called "the sabbath of Sabbaths". It is the day of atonement and a fast day. However, someone who is required to eat for health reasons, must eat. Jews who do not follow many religious practices still attend services on Yom Kippur, especially the evening services for Erev Yom Kippur.
Observance of Yom Kippur has many aspects. It is a day of fasting (not eating any food or drinking anything including water). It is usually a day of prayer and study.
As Yom Kippur is an extremely important day, there are many traditions around it. Some Jews will not wear leather on Yom Kippur. Some wear mourning clothes or even the ritual clothes that they plan to be buried in.
Yom Kippur falls on the 10th day of Tishri and is also the last of the "Days of Awe". It never occurs on a Sunday, Tuesday or Friday.