Actually, first dining is at 6. As mentioned, 4 pm is last call for boarding, you can get on board as early as noon or 12:30. Your luggage should be at cabin before the lifeboat drill at 4:30, or right after at 5 at the latest.
But to answer your question, in the broadest terms, I don't know how folks get ready for 6 pm seating on any day on a cruise either, that's why we always get late seating.
But be warned with Disney, late seating means EARLY show times, 6:30, at least for most shows. Some shows they do a 10:30 pm third performance, 10:30pm being "normal" show time on other cruise lines for late seating diners. One show, on a sea day on the Eastern had an early 2 pm showing.
While we found the early shows to be a pain, the days they had three shows we took advantage of the alternate show both times, fit our schedule better.