All "benefits" for WDW resort stays are good starting with first-thing in the morning on check-in date** thru close-of-business on check-out day.
** Use "on-line check-in" (in MDX account) to say that you will be arriving at, say, 7:00AM so that (even though you won't actually be "at" the WDW resort) your "benefits" that day will go into effect that early morning.