It normally starts the last Friday in May, which is the 29th. C4's schedules for programmes such as Fit Farm and Friends appear to finish about this time which leaves the schedules clear for BBLB and BB. Having said that the official BB site is asking for nominations for 50 best moments of BB so perhaps they will show that that week and start BB the week after.
If it starts on the 28th and runs for nine weeks as usual it finishes just as the kids break up for summer and the holiday season starts, so I would imagine they would want the final before then.
Hope this helps, but really I don't know any more than you!!