The rumor is late May/early June. But until confirmed by Disney, it is just a rumor. There are a few reasons to believe it's true though....certain tickets are blacked out for those dates, CM passes blacked out, a press event.
There's a couple threads about it on the rumors board if you want to keep up with the info.
There's some new restaurants opened at Disney Springs, and a new character breakfast coming to Boardwalk.
New Main Street Welcome Show and MK opening procedure.
There may be more but I can't think of any right now.