We would like to schedule our London trip next spring to see the pageantry of the celebration whether we are lucky enough to procure tickets for the bleachers or not. Some sites say June 17, 2017 but nothing is official as yet as near as we can tell. We know it's early to think about next June but we use FF miles and they go on sale soon.
this is official
It's always the third Saturday in June. Assuming Queenie lasts that long. Charles will be able to choose his own date, presumably.
Since Charles' real birthday is in November it probably will stay where it is. Prior to 1959 it was the second Thursday in June though; now it can be the first, second, or third Saturday in June depending on the monarch's other diary commitments. Edward VIII and George V had real birthdays in June.