Fifty years together is a massive milestone, and pulling off a surprise family dinner takes a bit of coordination. You want to lock down that private room three months out so you actually get the space you need without scrambling at the last minute.
Big family events usually come with a steep bill, sitting right around $1500 for a proper private setup. When you are looking at a lump sum like that, spreading the cost over time can make the family gathering much easier to swallow.
That 18-month intro window is a strict deadline, not free money, and any remaining balance after it ends will get hit with standard interest charges.