Savannah in October is about as good as the US South gets. The brutal summer heat finally breaks, the humidity drops, and walking the historic squares under the Spanish moss feels like stepping back a couple of centuries. You go for the architecture, the ghost tours after dark, and the food, which means you want your schedule open to wander rather than rushing from site to site.
A typical trip runs about $1800 for flights, a place to stay, and eating your way through the historic district. Dropping that much at once can sting, so spreading the cost out helps make it manageable without missing the best time of year to visit.
That 0% window is a deadline, not free money. If you still have a balance left when the 18 months are up, standard interest charges will kick in on what you owe.