Yom Kippur is on the 10th day of the Hebrew month of Tishrei.
On Erev Yom Kippur (the 9th of Tishrei), in the early morning we do the Kapporos Service. (See the Machzor - special prayer book for the days of awe for detail.)
Later on during the day we eat the festive meals, to demonstrate our faith and confidence in G-d's mercy.
Another beautiful custom for this day is that of parents blessing their children with the Priestly Benediction: "May G-d bless you and guard you... May G-d shine His countenance upon you and be gracious to you... May G-d turn His face toward you, and grant you peace.
Yom Kippur atones for sins against G-d, but not for wrongdoings between man and man. It is therefore important, on the day before Yom Kippur, to apologize and seek forgiveness from friends, relatives, and acquaintances, to heal any ill feelings which may have arisen.
Before the Mincha (afternoon) service it is a custom to give extra Tzedaka.