Yom Kippur is described in many ways. One very poignant description is that it is “once in a year.” You see, the human soul is also described in many ways, with five different names, each describing a deeper level of her being. The fifth, deepest level is called yechidah, which means “one and unique.” Yechidah is the soul as she is fused and one with her Creator, so that the two are an inseparable whole. Yom Kippur is the day that the essential bond of yechidah shines within the time and space in our world. Meaning that once in a year, the One Above unites with the essential oneness of the soul here below within each one of us. All else falls away. Hitvaadiyot 5747, Vol. 1, pg. 113. Hitvaadiyot 5750, Vol. 1, pg. 101.