A milestone retirement open house party is all about celebrating the people who shaped a career, so the last thing you want to stress over is the guest list or the venue capacity. Because you need to book the venue space a few months in advance, lead-time is your real lever for keeping costs around the typical US cost of about $850 under control.
When you map out your timeline early, you can figure out what needs to be locked in now and what can wait until the weeks leading up to the event. Breaking down the expenses means you are not stuck scrambling to pay for everything at the last minute.
Just remember that the intro window is a deadline, not free money, and any remaining balance after 18 months will start accruing regular interest.