JetBlue just extended their schedule to allow you to book flights through June 14, 2017. Up until this point you could only book through May 2, 2017. While most larger airlines allow you to book travel 330 days in advance, JetBlue extends their reservations systems a few months at a time. This means you can go ahead and book for Memorial Day and the rest of spring.
I know booking this far ahead of time is nerve racking as you do not want to book a flight and then have to change or cancel it – and those fees are not cheap! However, if you have JetBlue Mosaic status, you can always cancel and rebook your flight for no fee. Also for all other passengers, if your flight goes down in the next 14 days, you can get a credit for the price difference.