It's not an organized event, it's just a challenge... you run on Thanksgiving day, and every single day through the holidays, until January 1st. The only requirement is that you run at least a mile each day. You can run inside outside, however you like it (but no elliptical).
I did this, back in 2013 (and logged 80 miles... that's not likely this time!) It's a great way to exercise through the holidays, builds perseverance and is a lot of fun!
39 days of running... will you join us? =)