I think they are opening on the Thursday because it is a couple of quiet days before the weekend so if it goes like what it did this first day of season they have got less guests to dissapoint.
I think after they have done all they need to on Safari Skyway they should open it up so people can get a good view of the Zoo on Zoo only days not just in the main season.
This year I may begoing on some trips to Adventure Island in Southend so I will get some theme park visits But I miss going to Thorpe and the close season goes slow