May not help a lot of us, but as of this morning MCI has a lot of openings on Saturdays and Sundays in May. They didn't have these yesterday.

Update: As kklems pointed out, this could be an error in the calendar, as the Customs web site shows MCI only open Tues-Thr. After reading other posts in this thread, I would not book on these days.

Update #2 : Agent at MCI on 4/18 confirmed the Saturday and Sunday interview times in May are accurate. You can sign up without worry (and I did!)