The whole point of these celebrations is to prompt guests to visit this year. They will surely keep whatever they've got planned under wraps until after they've given up trying to sell folks on this celebration.
Having said that, we know that October 1 is the 35th anniversary of the opening of WDW, and next year is the 25th anniversary of the opening of Epcot. I believe Pooh is having some round-number anniversary soon as well.