A first car is less about the sticker price and more about what it costs to keep on the road. Insurance, gas, tires, the occasional repair bill — those are the numbers that actually hit your checking account every month. Late autumn is when used-car inventory gets a little softer, dealers are sitting on older lots, and private sellers start thinking about not driving through another winter in a car they don't want. That's the window worth aiming for, not because cars are cheap, but because people are more willing to negotiate.
The deal you negotiate and the way you pay for it are two different problems. The price of the car comes down in the showroom or in a private-seller text thread. The financing is a separate decision. A 0% intro window on a credit card is built for a deposit, a set of tires, the first insurance premium, maybe a mechanic's inspection — not usually for the whole car. Treat the negotiation and the payment plan as two meetings, not one.
The 0% window is a deadline, not free money. If a balance is still on the Citi Simplicity® card when the 18 months end, the regular interest rate kicks in on whatever is left, and it applies back to the purchase date. Set a calendar reminder a month before the window closes and pay it down to zero, or transfer the remainder to another 0% offer before time runs out.