Picture the room when the first song starts and the dance floor actually fills up. That doesn't happen by accident. Booking a live band for an anniversary party takes months of lead-time, starting with locking in your date well before the calendar fills up.
Good talent gets snapped up early, and you'll usually need to put down a deposit to hold the date. Knowing your timeline helps you figure out what to pay now and what can wait until the big night gets closer.
That 18-month window is a hard deadline, not free money, so make sure the balance is fully paid off before it ends or standard interest rates will kick in on what's left.