I've participated in this readathon before(though the one year I was unable to do), and find it to be fun. It's also really motivating while still being relaxed enough that you don't stress.
Here is the blog with the info on when it takes place this year. The link goes straight to reader sign-ups, but there's other ways to be involved too if you look at the home page. They also talk about the original readathon and how they've continued doing it in honor of that person.
I've already signed up and have started planning out some of my reads. Books I'm near finishing, low page count reads, and short stories are part of my plan. I'll list some of my reads later if anyone is interested.
Anyone else participating? What is your reading plan? Have you done it before? Have you done others?