I believe Jet Blue is 9 months out before you can book. So, for December travel I would guess sometime in March. I started seaching JB in June '09 for a December flight and found plenty of seats available. I was travelling mid-December though.
We bought our tickets with Jet Blue last year on May 8th for tickets on December 18th flying non stop Buffalo to Orlando- paid $72 each. I am pretty sure it was the day they were released for that time period as I had been checking EVERY day for a long time waiting and waiting.