Lisbon in the autumn is about crisp air, empty viewpoints, and late afternoons catching the light on centuries-old azulejos. October and November drop the summer crowds while keeping enough warmth to wander the Alfama and Bica hills all day.
A solid week running around $2500 for flights and a decent central apartment takes some planning. Spreading that cost out over time helps keep your monthly budget intact while you lock in your dates.
The 0% 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 the remaining amount.