A neighborhood garden party in the height of summer needs a solid plan by early spring. The best rental tents and tables disappear fast, so booking in March or April is your best bet to secure what you actually want.
Spreading your $600 costs over the months leading up to the event makes the numbers much easier to handle. You want to focus on the guests, not the bank balance.
That 0% window is a deadline, not a gift. If you still have a balance after the 18 months are up, the bank will start charging interest on the remaining amount, which can get expensive quickly.