A rented ice cream truck turns a birthday party into an instant neighborhood event. Booking on a weekday afternoon is your best move, as many vendors drop their minimum booking fees when they are not busy with weekend festivals.
You want to lock in your date at least two months out, especially if you have a specific truck in mind. Once the date is set, you can break down the payment to keep your monthly cash flow steady.
That 0% window is a deadline, not free money. If you have not paid off the $400 by the end of those 18 months, you will start paying interest on whatever remains, which makes the rental more expensive.