While this is a rare occurrence, it is possible that at an extremely busy time, Disney may temporarily suspend admission into the park for a few hours. Panelist Bret does a great job
HERE of describing how park capacity issues are handled during high-crowd times such as Christmas.
The bottom line is that if you're staying off-site and want to visit
the Magic Kingdom on one of the busiest days of the year (July 4th week
and the week between Christmas Eve and New Year's Day), you should plan
to be at the MK parking lot
at least
an hour before the park opens. You should also plan to stay for the
entire day, because once you leave there is no guarantee that you'll
get back in.
This park capacity problem is ONLY an issue on the handful of busiest
days of the year. At other times, off site guests will have no problems
visiting the Magic Kingdom, or any of the Disney theme parks.