Maria Droujkova made it up in 2009, as a birthday present to herself. September 25th is my birthday too, and I loved the idea. So now I'd love to get lots of presents - math stories you all write. Here's my post on Math Storytelling Day 2 years ago. (I must have been too busy last year?)
If you'd like some inspiration, read The Man Who Counted, The Number Devil, or Math Curse.
Poems are good, too.