First, a blog that has a master list of reading challenges.
These are the upcoming read-a-thons I am aware of/participating in. The first two require sign up posts, the third requires you to fill out a form.
COYER Winter Switch: December 16, 2017 - March 9, 2018 - two versions: regular COYER or COYER Winter Switch (I am doing the switch, which requires you to make a reading list/shelf).
Bout of Books Cycle 21: January 8 - 14, 2018 (sign ups aren't up yet on the blog).
24 in 48 Read-a-thon Winter edition: January 27-28, 2018 (form isn't on the blog yet).