To get on this, you'll need to have a fairly flexible schedule in September, as these deals apply only to specific flights departing at specific times between September 2 and September 26. And as is the case with most of these sort of JetBlue deals, there's some fine print worth mentioning. For example, the sale only applies to "Blue" fares, the airline's lowest-tier flight option, so if you want to check bags it's going to cost extra ($25 for the first, second bag is $35).
That said, it'd be tough to find a more affordable way to sneak in one last long weekend before settling in for long sleeve weather. Then again, considering the frequency with which JetBlue has been running these sales lately, you might have another chance to get away for cheap sooner than you think.