This past time I saw it was noted on the main page of the JB website about 3 weeks before they opened it up on Feb 5th. It was the first time I had ever noticed it announced that early, but I also was not checking in the previous releases so I donlt know if that is new or not.
The website flyertalk has, as they say, "A brief (and probably incomplete) history of schedule extensions from JetBlue:" here:
http://www.flyertalk.com/forum/jetblue-trueblue/1116766-jetblue-schedule-extension-thread.html
As you can see in the past they have opened it up the late Oct to early Jan schedule anywhere between mid March and early April but last year they waited all the way until June.
"25 March 2012: Schedule extended through 6 Jan 2013
3 April 2013: the schedule extended through 6 January 2014.
6 April 2014: the schedule extended through 5 January 2015.
15 March 2015: the schedule extended through 4 January 2016
5 June 2016: the schedule extended through 15 February 2017"