Once a thousand years ago, the pony god of death witnessed a horde of ponies pouring into his domain on the night of Luna's madness.  The cries of sorrow from the living world were more than even he could bear, so that next day he broke the rules. For a single day and night, the dead could walk the earth.

And so it continued, year after year. The day after Nightmare Night, the dead walk again.

"What do you mean, we missed Apple Bloom's cutie mark? Oh, darling, I'm so sorry."