From:
http://www.usual-suspects-sailing.com/boat-2yr.htm - which is a fascinating, well written, blog on owing a charter vessel.
"most of the peak winter season charter bookings are made by November 30. Gaps do exist in the bookings schedule at this point in the year. While the boat is eventually booked, it is often done under a promotional rate. This observation tells me to avoid booking" [your charter vacation] "until December, then book where gaps exist."