Most fast days are days of mourning for the Jewish people. When my children were young and allowed to eat I wouldn't let them have any treats on a fast day. "Something sad happened to the Jewish people today," I would tell them. "It's not appropriate to eat candy or other sweet snacks."
Not so with Yom Kippur. In fact on Yom Kippur the opposite was true. The children who were too young to fast were encouraged to eat something special. I wanted them to recognize that Yom Kippur is actually a happy day. I wanted them to understand that Yom Kippur is a day of joy and opportunity.
On other fast days, our lack of eating is a deprivation that emphasizes our mournful state.
On Yom Kippur we don't eat because we are like angels. We are soaring above the earth, living as souls, not tethered to the needs of our bodies.
On Yom Kippur, we are given a glimpse of who we can be if we don't fall back into familiar negative patterns and behaviors. Just for that day we transcend our corporality and touch the infinite (or at least we try to).
And we experience joy – the joy of knowing there are no limits to what we can accomplish and how much we can grow, the joy of knowing that we are not trapped by our past actions, the joy of the chance to start anew, the joy of our fresh beginning.
All the mistakes we made in the past are extinguished, are nonexistent. We can make fresh commitments (small ones) and new decisions (also small ones).
So it's a happy day. A cleansing day. A spiritually auspicious day. We know all too well the pull of routine, the inertia of habit, the rut of our usual patterns. This is our opportunity to break free, to (quoting a certain world leader) "push the reset button."
This is a day of joy because it is such a gift to us, such a kindness from our Father in Heaven. It is day of joy because it wards off despair and encourages hope. It is a day of joy because tomorrow we will really be different.