Usually not until their busy season has ended - then they start production, and often we're sent brochures around Christmas time. However, they're happy to accept bookings any time, and some of the best sites may be booking up already. Don't wait until next year's brochures start to appear - prices and opening/closing dates probably won't vary that much from those displayed on the internet for this year. If you want to pre-book then as long as this year's price doesn't seem too bad I think you'd be quite safe - and if you're going outside peak season (mid-July to mid-August) it probably isn't necessary to book anyway.