The Resort Airline Check-in desk at each resort opens at 5:00 a.m. The transaction needs to be complete by no later than 3 hours before flight time, so therefore, unfortunately, any flight that departs before 8:05 a.m. or so is ineligible to use RAC.
It’s NOT possible to use RAC the day before your flight; it’s against TSA regulations. The RAC desk closes daily at 1:00 p.m. anyway.
And by the way, don’t go looking for a “Disney’s Magical Express” desk at your resort, there won’t be one. Resort Airline Check-in has nothing to do with the DME bus service