A part of me thinks if Universal was smart they would just announce the opening date the day before it opens. I mean, if you think about, I think they REALLY want to avoid what happened last time. With Phase 1, they had a ton of soft openings, videos of the land and ride posted all over the web, and then since everybody had seen videos of it and knew the date well in advanced, everybody and their grandma showed up on opening day which was a nightmare for EVERYBODY involved.
If they keep things secret, the Grand Opening will not be nearly as insane as Phase 1. Press comes down on the 17th and during the first night event they break the news to the press and tell them all to run a story saying "Diagon Alley opens TOMORROW!" on the 18th. The locals that have off work that day and some insane people willing to pay last minute airline fees will be the only people attending the Grand Opening besides the people that just happen to be there. On top of this, the general public will then book their trip since I think a lot of people that want to visit are holding back until a date is announced. If they announce a date on June 18th, that will give people enough time to request off work and make it down before summer is over. People will not all arrive on the same day so things will be a little more spread out.
Universal knows for a fact this area will be a MASSIVE success just like Phase 1 no matter when they announce the opening date. People will be coming from all over the world for years to experience it. Instead of making day 1 a nightmare, keep things a secret until the last possible moment and then the day 1 rush won't be nearly as bad.