Late September is the sweet spot for Greece. The Aegean is still warm enough for swimming, the brutal summer crowds have cleared out of Santorini and Naxos, and ferry rides between islands are actually peaceful.
A solid trip runs around $3,800 from the US. Instead of draining your savings all at once, you can map out your bookings and use a dedicated zero-interest window to pay it down steadily over time.
The 18-month window is a firm deadline, not free money. If you carry a balance past the end of the intro period, regular interest rates kick in on the remaining amount, which will quickly make your vacation more expensive.