The past few times, prices have increased at the beginning of the year, and with practically no warning in advance. In previous years, though, some, but not all, admission prices were raised at the beginning of October (again, with no warning in advance).
The price increases have been so minimal in recent years that at this point I'll take my chances and buy my February admission in December rather than August. That gives me (and my guests) that much longer to save up for them.