Yom Kippur is the holiest day of the year -- the day on which we are closest to G-d and to the quintessential core of our own souls. It is the "Day of Atonement" -- "For on this day He will forgive you, to purify you, that you be cleansed from all your sins before G-d" (Leviticus 16:30).
For twenty-six hours, from several minutes before sunset on Tishrei 9 to after nightfall on Tishrei 10, we "afflict our souls": we abstain from food and drink, do not wash or anoint our bodies, do not wear leather shoes, and abstain from marital relations.
The day is the most solemn of the year, yet an undertone of joy suffuses it: a joy that revels in the spirituality of the day and expresses the confidence that G-d will accept our repentance, forgive our sins, and seal our verdict for a year of life, health and happiness. When the closing Ne'illah service climaxes in the resounding cries of "Hear O Israel... G-d is one" and a single blast of the shofar, the joy erupts in song and dance (a Chabad custom is to sing the lively niggun known as "Napoleon's March"), followed by the festive after-fast meal, making the evening following Yom Kippur a Yom Tov (festival) in its own right.
May you be inscribed in the Book of Life.
May you have a meaningful and easy fast.