To play the Devil's advocate here, I think I can see where Brian is coming from. Obviously, sub-processes are on a pretty tight schedule. The contractor really needs to know for how many days he can rent the cranes, and the crane owner definitely likes to know when his cranes will be finished on that project so he can rent it out to somebody else. But it's impossible to plan out a project like this with perfect accuracy, you have to have some leeway, especially for sub-processes that will only start in the future. You can't tell the paint contractor half a year in advance that he has to show up with eight people on January 8 and work for six days, because if something happens that pushes completion back two days, that paint contract will have to be re-negotiated.
I can believe that their project delivery model goes somewhere along the lines of "When process A is more than halfway complete, we set the dates for initiation of process C". For example, process C may take a week, but is currently only scheduled within one month. Not sure if it is the first week of January or the third week, but sometime in January sounds pretty certain, and when process A is finished they will know which week.
Currently, the main contractor for the "raw building" is working on his tight schedule, but I bet it's still unclear when the pull-through tests will occur, for instance. That may be determined when the coaster is more or less finished and the contractor has moved out. But they have probably scheduled what will happen in the week the contractor finishes his work, perhaps carpentry work for theming details will begin.
The end result of all this leeway is that they haven't determined the opening date yet. Opening month, probably, target opening week might be more or less certain by now, but no date can be promised yet. For instance, it's very awkward to print 20,000 flyers displaying an opening date of May 20, and then hit a delay that pushes the entire process back by a week. Then you're stuck with boxes full of useless flyers, and you have to order new ones. Sometimes, it's best not to promise any dates until you've ironed out any uncertainties. With a complex construction project, there will be a lot of uncertainties.
I know the LEAN model of construction is pretty wide-spread by now, and that process indeed begins with a determined end-date, but I can totally see Phantasialand running a different delivery model. They're not a company specializing in projects like this, so they probably don't employ personnel trained in LEAN. It might be simpler for them to choose a different project model.