As expected, JetBlue just extended their schedule to allow you to book flights through June 12, 2019. While most larger airlines allow you to book travel 330 days in advance, JetBlue extends their reservations systems a few months at a time. Up until now you could only book through April 30, 2019, so this means you can now book for all of May, Memorial Day Weekend, and half-way through June! 
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 5 days, you can get a credit for the price difference. Related posts: JetBlue Flight Went Down in Price, Get a Credit Back; JetBlue Went Down in Points, Get Your Points Back
Remember, you have 24 hours to cancel a flight after it is booked for a full refund if you want to “book now and figure it out later.†If your plans are flexible, use the JetBlue Fare Finder which will give you the cheapest price for every day of the month.
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These fees for rebooking look astronomical, would be useful to include some detail on that. Booking far out wouldn’t be for me unless it was so astonishing cheap I was prepared to just forfeit the fare.