Signup at the link which also explains:
"If you’re new to 24in48, this is the basic gist: beginning at 12:01am on Saturday morning and running through 11:59pm on Sunday night, participants read for 24 hours out of that 48-hour period. You can split that up however you’d like: 20 hours on Saturday, four hours on Sunday; 12 hours each day; six four-hour sessions with four hour breaks in between, whatever you’d like.
And that’s it. The format never changes but it’s always an adventure."