If you just want to catch whatever Stephen Colbert does at the Correspondents' Dinner tonight, C-Span is currently saying that the post-meal program will commence around 9:30 PM (all times Eastern), George W. Bush will speak at 10:05 and the Prince of Truthiness will go on around 10:25. Right after the dinner is concluded, C-Span will re-air the entire thing and they're saying that will start "around Midnight," plus they'll run it all again on Sunday at 12:30, which I assume means 12:30 PM.
That's what they're currently saying on-air. The schedule on the CNN website says something else. This is not unusual for C-Span.
Also, I believe Mr. Colbert is the subject of a story tomorrow night on 60 Minutes.