[Treble Hook Jim]

Here is a thought... Have you actually contacted the captain?
Based on you description of the situation, you have not.
It is likely possible that the 7th man has not actually booked the date but would like to and is trying to get people together.
Before you toss the captain under a bus and get your panties in a wad about this, contact the captain and book your requested date that you talked about previously and if he advises the date is not available, then advise him of your past bookings of this date and your conversation. If he does not let you have your date... There are plenty of really good other boats who will put you on fish. Likely the 7th man does not have this booked with a deposit yet if the capt does not take reservations until January...