I recently won a JetBlue Travel Certificate for a free roundtrip flight anywhere JetBlue flies in a Twitter contest they were running. The terms of the Travel Certificate lists all blackout dates for 2016 and 2017 (basically all holidays) and also says that availability may be limited during peak periods, such as Fridays-Sundays in summer, and to warm weather destinations in winter.

I'm wondering what determines what flights are available for use with the travel certificate? I think you can only find it by calling in, but am I looking for a certain fare class or something?