Late September is the sweet spot for Spain. The brutal summer heat finally breaks, the crowds at the Sagrada Familia and the Prado thin out just enough to breathe, and you can actually enjoy a long afternoon on a terrace in Madrid without melting. You get crisp walking weather, lower-stress booking windows, and the kind of unhurried pace that makes a two-city trip feel like a real break instead of a checklist.
A typical US traveler drops about $3800 on a trip like this once flights, hotels, and daily spending are all added up. Spreading that cost out helps you lock in your dates early while keeping your regular monthly cash flow intact.
The 0% intro window is a deadline, not free money. If you still have a balance left when those 18 months are up, standard interest kicks in on what you owe.