More than likely, they won't tell you because they don't know themselves.
It's because summer English camp isn't arranged ahead of time, usually it's about 2-3 weeks before the start date.
If you're really lucky, you may get a month or two notice to start planning for vacation, but don't get your hopes up!
I'm on my third contract, and my summer/winter vacation dates have been set already. Try to talk emphasise it's important that you know these dates well ahead of time.