“To everything there is a season … ” we’re taught, “a time to be born and a time to die; a time to plant and a time to pull up what’s been planted …” (Ecclesiastes 3:1-8) — and an express time for repenting for our sins and returning to G-d. And that time is Yom Kippur. For while repentance is always possible and invariably effective, Yom Kippur is the most favorable day of the year for it.
All our sins are annulled when we repent on the holy day of Yom Kippur, all the metaphysical harm they’d done (to others and ourselves) is undone, and all the pitch and murkiness we’d brought on as a result of them is lifted. And an express celestial light is revealed on that day that allows us the wherewithal to draw close again to G-d Almighty and to enjoy the holy equipoise that comes from that.
But that special light only irradiates when we fulfill the mitzvot specific to that day, most especially fasting. For when we do that, we remove ourselves as far from physicality as we can on this earth and we ascend heavenward for the day.