Retirement parties move fast once the date is set. You want the food to be great and the mood to be right, but dropping a grand on catering can sting if you do it all at once.
Booking your caterer one month out gives you enough lead time to lock in a menu and secure your date. By spreading the cost, you can cover the brunch without stressing your monthly cash flow.
Remember that the 18-month window is a deadline, not a gift. If you still have a balance once those 18 months are up, you will start paying interest on whatever is left, so treat this like a strict monthly bill.