We can learn the key lesson of Yom Kippur from something Harry Potter tells the evil Voldemort in the final battle at the end of Deathly Hallows:
"Before you try to kill me, I'd advise you to think about what you've done ... think, and try for some remorse ..."
"What is this?" [Voldemort replied.]
Of all the things that Harry had said to him, beyond any revelation or taunt, nothing had shocked Voldemort like this. Harry saw his pupils contract to thin slits, saw the skin around his eyes whiten.
"It's your one last chance," said Harry, "it's all you've got left ... I've seen what you'll be otherwise ... be a man ... try ... try for some remorse..."
Harry's point, which Voldemort may have understood at some level, is that remorse was the only way that Voldemort could save his soul, which had been damaged by all the dark magic and evil that he had done. He learned this lesson from Hermione in chapter six, when the three of them are learning about destroying Horcruxes:
"And the more I've read about them," said Hermione, "the more horrible they seem ... It warns in this book how unstable you make the rest of your soul...."
Harry remembered what Dumbeldore had said, about Voldemort moving beyond "usual evil."
"Isn't there any way of putting yourself back together?" Ron asked.
"Yes," said Hermione, with a hollow smile, "but it would be excruciatingly painful."
"Why? How do you do it?" asked Harry.
"Remorse," said Hermione. "You've got to really feel what you've done. There's a footnote. Apparently the pain of it can destroy you. I can't see Voldemort attempting it...."
This, in a nutshell, is the lesson and purpose of Yom Kippur. The day is designed for us to feel remorse, and by doing so we have the power to repair our souls, to clean them back to their pure state.
We start with Kol Nidre, where we express remorse for any vows or promises we may have made, intentionally or not, during the year. We continue with repeated renditions of the vidui prayer, hitting our chests while confessing all the types of bad things we may possibly have done. We recite "Avinu Malkeinu," our Father our King, "we have sinned before you." And we pray for G-d to accept our remorse and wipe the slates clean, giving us a fresh start for the new year.
The midrash I've quoted previously states that if we truly feel remorse during the time from Rosh HaShana to Yom Kippur, G-d will judge us as brand new creations, with clean slates.
Jewish Law also tells us that our remorse should start with things that we may have done to other people. With that in mind, I'd like to ask that anyone reading this, people I know or don't know, who I've bothered or offended in any way whatsoever, please forgive me. And I'll say publically that I forgive anyone who's done anything at all to me.
As Hermione said, true remorse can be painful. We don't want to admit things we've done, we're conditioned to make excuses for them. But on Yom Kippur, remorse is the theme of the day. And at the end of the day, the slates are wiped clean, and all mistakes are erased as if they never existed. As painful as remorse can be, the goal is a positive one -- the chance to start again, to start the new year as pure as newly created, with no baggage, only opportunity.
As Harry said, this is the chance we have for our souls. Let's use it!
I hope everyone has a meaningful Yom Kippur and as easy a fast as can be meaningful.