What is Hearth's Warming Eve all about?

Is it about friendship? Is it about forgiveness? Is it merely about unicorns, pegasi and earth ponies being brought together in love and harmony?

For Celestia and Luna, there is much more to it than that. For them, it carries a tradition, one that even through the darkest of times, retains its significance.

Nopony knows what it is, and why it is so important.

But snowpony... snowpony knows.