Retirement parties are all about the moment, but the planning usually starts months before the first toast. Focus your energy on picking a venue that fits the vibe and locks in the date early so you aren't scrambling when the calendar fills up.
Since you are aiming for a weeknight, you have more flexibility with venues and pricing than on a weekend. Treat this as a project with a fixed finish line to keep the costs from creeping up.
Remember that 0% interest is not free money, it is just a grace period. If you still have a balance once the 18 months are up, you will start paying interest on whatever is left, so stick to your monthly plan.