I've said I was going to do this ever since I started NaNoWriMo. Since a lot of us don't finish our novels during the actual month, and it's so common to start a book and never finish it, I thought it would be fun to have a month where we pick a manuscript that we've started somewhere along the way (doesn't have to be a nano) and finish the sucker. My vision for this is to work on a MS that actually needs writing to be done on it rather than just editing, and maybe have a National Novel Editing Month later.
Chris Baty is having a drive to finish your novel this year, taking the whole year to do it, but for me that's too long. I need the pressure of doing it in a month, and it's helpful to have people doing it with me. I'm thinking January or March, giving us time to recover from NaNo and the holidays, but giving us 31 days.