You want that park pavilion locked down on the very first day bookings open so you actually get the date you need. Waiting around means losing the spot, and then you are scrambling to find a backup space for a crowd of teenagers and family.
A typical US graduation party at a rented park runs about $800 once you factor in the shelter fee, food, and basic setup. Spreading that out over time makes it much easier to handle while you are buying decorations and supplies.
That introductory window is a strict deadline, not free money, and any leftover balance when the 18 months are up will suddenly start collecting regular interest.