Special case: Last-minute extensions at the MK sometimes happen mid-way through the day based on the morning's gate, with no prior notice. When we were there last May, it happened on 3 days of our 5-day visit.
When this happens, the info is usually not posted except at the parking entrances and on the tip board at the entrance to the park. If it seems especially crowded, check with CM's around lunchtime to see if hours have been extended. If you are at a different park, ask a bus driver; they always know.