I believe,
but I could easily be wrong, that dates will only be available to book one month ahead of time, i.e. a Jan assessment day will only be released for booking in December. This is because most pilots only get their rosters one month in advance and BA want to avoid masses of people phoning up wanting to change dates when their rosters are released if they had been able to book assessments 2 or 3 months in advance.
Regards