Focus on the surprise and the day itself. When you are planning for a milestone, the park permit is your first deadline. You need to secure your spot months out, which anchors the rest of your timeline.
Spread out the spending to keep your bank account balanced. Since the date is fixed, you can map out every vendor payment leading up to the event.
Remember that the 18-month window is a deadline, not a gift. If you still have a balance when the period ends, you will start paying interest on the remaining amount.