Flights are available on select days from the middle of September through early November.
Searching on the JetBlue site, I found seats departing JFK on Saturday, September 15 and returning from GND on Tuesday, September 18, 2018.
Have a look at our New York (JFK) flight deal page for a full list of airfares we've found today.
Need a place in Grenada? Here are a couple options for the listed dates on Hotelwatchdog.