1) At ANY resort, check-in can be done at 6:05 am on your arrival day.
2) The computers change/update at 2:30am-4:00am.
3) But you can't check-in earlier, because it is like getting another night free.
4) Example:
. . . make a reservation for Tuesday night check-in
. . . ressies are actually for the night, night, and not for the "day"
. . . check-in on Tuesday at 5:00am
. . . sleep for 3-5 hours before going to the parks
. . . effectively, you checked-in on Monday night
. . . and, yes, people have done this in the past