Usually they are not posted until they are official and parks have informed the local regional reps. Some parks will announce right after the previous year, which is why you can see some already on the calendar for late in the year. Others wait a bit longer to inform ACE. But once the information is known, they are usually put up on the calendar immediately. So yes, they get added as the season progresses. If event dates had already been finalized, they would be up there.