Yom Kippur

Why Yom Kippur? What is unique about Yom Kippur? What makes it the holiest day of the year? We all know what we do on Yom Kippur; we fast, we spend almost the entire day in shul davening, beating our chest reciting ashamnu and al chet, and singing moving, soulstirring songs. But what makes Yom Kippur dierent from any other time?