I'm new to this board but i think they do it from holiday to holiday. Last one was a labor day challenge i think the next one will be a short halloween one after columbus day. I would jump into those challenges!
I would be up for a longer New Years' Challenge. New Years' Eve will be my one year mark and originally when I wanted to hit goal... which is still possible, but not if I don't stay on track and kick it back into gear with my exercising. Let me know if you start one up and I'm in!