Contrary to many people's perception of Yom Kippur as a sad day, the Sages describe it as one of the happiest days of the year!
Although Yom Kippur is certainly a solemn day of self-reflection, it is also a day of happiness. It is a special day when we can become free of the shackles of our past, when God gives us a special opportunity and ability to repent and change ourselves for the better. While on Rosh Hashana we look forward to and pray for a good new year and commit ourselves to being better people, Yom Kippur focuses us on our past and gives us a chance to "turn back the clock."
Yom Kippur is known as the "Day of Repentance". Through fixing anything we have done that's wrong we can actually erase it and start the New Year with a clean slate.
May we all be written and sealed in the Book of Life!